Fathomless Warlock 5E Guide

Tasha’s Cauldron of Everything introduced a ton of classes with specific utility and without too many downsides. The Fathomless is one of those patrons! The Fathomless is an entity, deep in the oceans. There is a nearly infinite amount of water in the world, rarely explored by anything but it’s denizens. This might be from the elemental plane of water, a god of the depths, or another terrifying, deep-water creature. Its power allows you to call upon the ocean’s strength, Aquaman-style, to decimate your foes. Is this going to be useful when delving into dungeons? Check out our Fathomless Warlock 5E guide to find out!

Embrace the Depths: The Fathomless 5E Warlock

The Fathomless is a combat-control warlock subclass with a ton of versatility. Their main mechanic is the Tentacle of the Deeps, which is limited to your proficiency bonus. Other than the tentacle, you gain some water-based versatility, some teleportation, and great spells to help you fight without your tentacle.

Expanded Spell List

The Fathomless spell list is pretty fantastic and versatile, with a mix of damage, disruption, and utility.

Create or Destroy Water is probably too specific. You can theoretically survive in a desert for a long time with the ability to create water at will, but the ability doesn’t scale well. Using your precious Warlock slots for 10 gallons of water is rough! Thunderwave is a good defensive spell to get creatures off of you and scales with d8s, which is solid.

Gust of Wind is another great defensive spell, which forces encounters to move in your favor. Again, not great for blasting; use this only if you think forcing creatures to take double the movement action is important. Silence, however, can be a devastating counter to any caster who is unable to move. Lock them down with angry melee allies and then silence them, and watch their world fall apart!

Lightning Bolt is a fairly basic damage spell with good range and good damage. Use it when lines would be most useful, and watch as 8-10d6 tears through your enemy’s lines. Sleet Storm is another combat-control spell, mostly to prevent enemies from really messing with you. It’s also an alright anti-concentration spell, though you’d rather hurt the person a lot instead.

Control Water is exceedingly good… if you’re near water. Part Water can invalidate water combats, Redirect Flow can solve environmental problems, Flood can cause elemental problems… But all of these require a source of water in some way. Otherwise, this is a waste of a guaranteed spell. Summon Elemental is a fine spell, getting Acid resistance and Amorphous form. I wouldn’t say Water is the best of the forms, but it’s good enough. The damage it deals as a bonus action is well worth it, too!

Bigby’s Hand is yet another complex spell with a ton of options. It’s not the greatest damage, but Forceful Hand, Grasping Hand, and Interposing Hand are all crazy strong. And 4d8 as a bonus action is still plenty! Bigby’s Hand is a great option if you want a summon that’s got loads of options and is nearly impossible to put down quickly. Cone of Cold is a huge area of effect spell that doesn’t do… much damage. Lightning Bolt does 10d6 (average 35) to Cone of Cold’s 8d8 (36). However, the huge range, cold damage, and larger radius makes it a very good choice for larger encounters.

Overall, really complex but solid spell list. Obviously, you’re best near water, but you have enough spells that you don’t need water to be useful. That’s a theme for most of the class!

Tentacle of the Deeps

This class starts with a pretty cool feature. You get to summon a tentacle.

As a bonus action, you create a 10-foot-long tentacle at a point you can see within 60 feet of you. The tentacle lasts for 1 minute or until you use this feature to create another tentacle.

When you create the tentacle, you can make a melee spell attack against one creature within 10 feet of it. On a hit, the target takes 1d8 cold damage, and its speed is reduced by 10 feet until the start of your next turn. When you reach 10th level in this class, the damage increases to 2d8.

This tentacle can move 30 ft and attack as a bonus action every turn, but you can only summon it a number of times per long rest equal to your Proficiency.

Not bad! This is a pretty solid bonus action; it’s like a summon but can’t be targeted. That’s a good and a bad thing; it can’t be taken out of combat, but it can’t necessarily take hits for you. At least it slows down enemies by 10 feet, which can prevent you from taking damage from melee opponents. The damage it deals is pretty abysmal, even early on. At least you have a good way to spend bonus actions whenever you aren’t casting hex?

Pop this during fights where you feel like having this tentacle in the enemy’s backline might be useful, and you don’t plan on casting other bonus action spells. If you do things right, you should have access to a potent bonus action-using spell every single fight! 

Gift of the Sea

You get another benefit at level 1! Well, technically.

You gain a swimming speed of 40 feet, and you can breathe underwater.

Awesome! This is one of the few archetypes that, at level 1, can start the game as an underwater class! If you ever want to do an underwater campaign, this ability alone lets you play as any race! 40 ft swim speed is also a little higher than average, so that’s a good swim speed.

Unfortunately, most campaigns aren’t underwater. You’ll be the best one to swim into a pool of water to find loot or solve a puzzle, but otherwise, this probably won’t come up often.

Do remember you have this; you’ll save the party a ton of headache if you can do underwater stuff quickly.

Oceanic Soul

At level 6, you get two more abilities! This warlock starts with some utility and defense;

You are now even more at home in the depths. You gain resistance to cold damage. In addition, when you are fully submerged, any creature that is also fully submerged can understand your speech, and you can understand theirs.

Cool! Cold resistance is wonderful, especially if your campaign is underwater. Cold damage is relatively common; not like Fire, but you’ll get some use out of this. It’s common for spells, and a ton of enemies have some on-hit abilities that deal cold damage. You’re not gonna be too sad about this!

Fully submerged Tongues is strong, but not too strong. You can overcome language barriers by diving into the water, but that’s not going to be useful. It does say any creature; that means you can finally start talking to fish. Most DMs will let you convince more intelligent water creatures to do actions… And maybe, you can convince less intelligent ones to help you out too!

This ability is more useful in underwater-focused campaigns, but you can do some weird strategies to overcome language barriers. Just make sure your Persuasion is high enough to explain why you dunked your conversation partner into a lake!

Guardian Coil

Also at level 6, your tentacle improves.

When you or a creature you can see takes damage while within 10 feet of the tentacle, you can use your reaction to choose one of those creatures and reduce the damage to that creature by 1d8. When you reach 10th level in this class, the damage reduced by the tentacle increases to 2d8.

The Warlock’s reactions are pretty limited, usually just to spells. So, this is a pretty consistent, strong reaction that can really help your frontline! Place a tentacle near a melee ally, spend your bonus action to hurt someone, and then you even get to reduce damage by a bit! d8 to 2d8 isn’t too impressive, but it might be all that’s needed to save a life.

Considering the Warlock doesn’t have a highly impressive list of reactions, this is fine. Keeping your melee characters alive is important, and gives you a reason to go to the frontlines yourself, if you wish. This is just good for your tentacle!

Grasping Tentacles

At level 10, you gain another concentration spell.

You learn the spell Evard’s black tentacles. It counts as a warlock spell for you, but it doesn’t count against the number of spells you know. You can also cast it once without a spell slot, and you regain the ability to do so when you finish a long rest.

Whenever you cast this spell, your patron’s magic bolsters you, granting you a number of temporary hit points equal to your warlock level. Moreover, damage can’t break your concentration on this spell.

Evard’s is a 4th level control spell. You essentially cast a small entangle, which restrains enemies and deals 3d6 damage. After they fail the initial save, they can only make Strength or Dexterity checks (not saves) to escape. This can really mess with a battlefield, and you can cast it for free! That’s really useful, and doesn’t take your bonus action, letting you use your other tentacle.

If that wasn’t enough, you also gain some temp hitpoints. 10-20 temp HP can save your life, but rarely soak enough damage to be noticeable. You’ll be lucky if an attack does 10 damage at this point! However, every hitpoint counts. 

And hey, your concentration spell can’t be canceled by damage! This is a safe option that’s even safer. Just… try not to die. Use this spell to keep you safe, and don’t put yourself into a bad position just to try and ignore damage.

Fathomless Plunge

Your final ability is really unique! At level 14, you gain access to this action.

As an action, you can teleport yourself and up to five other willing creatures that you can see within 30 feet of you. Amid a whirl of tentacles, you all vanish and then reappear up to 1 mile away in a body of water you’ve seen (pond size or larger) or within 30 feet of it, each of you appearing in an unoccupied space within 30 feet of the others.

This can be used once per any kind of rest.

This is a pretty niche ability. A mile is a fantastic range for a teleport, but requires you to know about that body of water. Check around for water before entering enemy territory, and you have a one action, no preparation required escape plan! 

Even when you’re done with the dungeon, this teleport doesn’t cost any spell slots or anything. You just get to… do it. So, if you have any reason to get to a pool of water nearby, you should use this ability!

If you have some caster with divination abilities, you might be able to more easily find pools of water. Then you have options to teleport to. The pond size restriction keeps you from teleporting to towns without natural water sources, which is a little bit sad. Ponds aren’t too big though, so you might want to see what your DM considers to be a pond before you start marking pools of water to zap to.

This is a great emergency teleport with a tiny bit of use as a simple movement ability.

Pact Boon Synergies

In my opinion, the two normal options for the Pact are good for you.

Pact of the Blade

Pact of the Blade has great flavor; you can grab a trident and become Aquaman incarnate. However, the base Warlock (nor the Fathomless) gives you good enough options for melee combat. You are much better off using Eldritch Blast and keeping your distance. The Dexterity requirement is not worth the invocations.

Pact of the Chain

Pact of the Chain tends to be a fantastic choice, and it remains a good choice here. If you want to dominate the early game, the Imp familiar will crush encounters with their insane Poison ability. They are also great scouts, which can give your party a ton of information! You can give your senses to the familiar to look for ponds, if that’s what you really want.

However, you have a problem. If you’re doing an underwater campaign, none of the Pact options of familiars can breath underwater. The normal familiars are… fine? But you really take Pact of the Chain for the incredibly powerful familiars. If you’re doing an underwater campaign, you should talk to your DM and consider other options that are more beneficial. Otherwise, take a pass here.

Pact of the Talisman

The Pact of the Talisman doesn’t offer too much for a Fathomless Warlock. It’s nice to have a permanent Guidance, but Pact of the Tome handles most of the Fathomless’s out-of-combat needs. We don’t want to dedicate ourselves to Talisman as a non-support class. So, best to get something slightly more useful, like Chain or Tome, to help us.

Pact of the Tome

The Book of Shadows tends to be the safe choice. If you want to be good in the late game, you pick the pact of the tome. You start with a ton of cantrips, which increases your utility, and then you can use invocations to get great utility spells. It’s a basic choice, but you’re a spellcaster at heart, and you’re going to want some spells that don’t take concentration, because you have so many that do!

Best Race for The Fathomless Warlocks

The Fathomless Warlock is looking for high Charisma for good spell attack rolls. Then, boost your Constitution and Dexterity to keep yourself alive, or your Wisdom if you’re worried about spellcasters.

Admittedly, Human or Half-Elf is the more optimal choice… But we chose Changeling, since it’s a unique and fun build with a ton of potential!

Changeling

Eberron: Rising from the Last War brought us the Changeling, which was errata’d in Tasha’s Cauldron to no longer give +3 Charisma. That’s fine! As a Changeling, you’re wanting +2 Charisma, +1 Constitution to boost your health, or +1 Dexterity to increase your AC. Shapechanger is not a hugely useful ability, but it can be a great problem solving tool for any social encounter. Besides, it combos well with Changeling Instincts, giving you two skills between all the Talking skills and Insight. Take Persuasion and Deception to be the trickiest fisherman, and convince fish of all sides to join your escapades!

Best Feats for Fathomless Warlock

The Fathomless Warlock, much like many Warlocks, should be eager to get feats that make the party like to take short rests with them. They should also take some feats to help improve their spell list significantly.

Chef

The Chef feat does something we really value in a feat: +1s to ability scores. Rounding out odd ability scores is one of a feat’s best features. It lets us be more aggressive during character creation with the promise to give us our modifiers later on. For Chef, the choice is between Constitution or Charisma, two extremely strong options no matter what.

Then, you get the fun stuff.

Cook’s Utensils are a fun, if not strong, artisan’s tool that could offer opportunities for you in more social campaigns. The more important aspects of these feats are the two short rest abilities that you get. This is just a bonus!

During short rests, you add a d8 to any healing that party members do with hit dice. An average of 4.5 might not seem like much of an addition, but it stacks up over time. In a party of 4 for instance, this feat can provide 18 health. If you short rest three times in a session, that’s almost 60 extra HP. Far from terrible.

And, that’s not the only thing you get for resting. You also gain treats, little bonus actions that provide a small amount of temporary hitpoints. Since you can also eat them outside of combat, these realistically provide your frontliners with quite a bit of a shield per fight. At level 17, this is 36 HP per rest, including after your long rest. Hope everyone’s hungry, because that’s 24 treats during a day where you short rest three times. 144 hp in total! Great, fun feat that’s worth trying if you have odd Constitution or Charisma during character creation.

Eldritch Adept

The Warlock’s invocation list is a mysterious but powerful source of spells known. Eldritch Adept is simple, in that it lets you learn an Eldritch Invocation. For a Warlock, this might seem a bit overkill.

However, your ninth Warlock Invocation can be swapped out separately from your standard array of invocations. This lets you use your ninth invocation slot as sort of a “problem solving invocation.” That can mean getting Eldritch Spear when you know you’re about to go into an open-field war, or get Polymorph before a humanoid boss with poor Wisdom saves.

This swap-out isn’t exactly easily achieved, but can help solidify a spell list before a dungeon. Or you can use it to get useful permanent abilities, like Beast Speech or Otherworldly Leap, before encountering specific challenges.

It’s not exactly a perfect feat, but it has uses for the Fathomless Warlock that can be exceptionally useful.

Fey Touched

Fey Touched is one of the strongest feats in the entire game, as long as you’re a spellcaster.

To begin, for our Fathomless friend, we get a +1 to Charisma. While not as strong as a +2, we’re still inching our way towards 20. And, if we start with 17 Charisma for whatever reason, this feat gets us to 18. That’s a +1 to our modifier, technically.

However, that’s just the tip of the iceberg. This feat also lets us learn two spells, a first and second level spell, and lets us cast them with our normal spell slots.

The guaranteed spell that we learn is Misty Step. This is one of the greatest spells in the game, letting us zap to a new location as a bonus action. That lets us set up for an Eldritch Blast turn while guaranteeing our safety. In niche cases, it can also help us solve puzzles by getting us somewhere that the DM doesn’t want us to get to. Having this learned permanently is a gigantic boon.

Then, you can learn another first level spell. This part is a bit weaker for a Warlock, since they cast all spells as if they were 5th level. However, the Warlock has a few options here that are quite powerful. Silvery Barbs is a decent reaction that protects the party, Bless can target multiple people at the cost of Concentration, Command can also target multiple creatures, etc.

This feat is great for extending your spell list and ensuring you have some defensive options. While not as strong as it is on other casters, you’ve got a great option here.

Inspiring Leader

Inspiring Leader is probably a very high priority for your standard Warlock build. Whenever you complete a rest, long or short, you can give a ten minute speech. This speech gives your party members a bubble equal to your level plus your Charisma modifier. That’s really something!

Let’s do some math. At level 20, a Warlock with 20 Charisma is giving 25 temporary HP to each party member, including themselves. If each Party Member expends their temporary hitpoints over the course of the next few fights, that’s an average of 100 HP blocked per party member. And let’s say you take three short rests over the course of the day? That’s 400 HP.

Inspiring Leader is a great way to incentivize party members like Wizards to take short rests, even if they don’t get much out of it. Which is great for you, since you get everything you need out of short rests.

Resilient

The Fathomless Warlock has the ability to ignore concentration checks whenever they cast Evard’s Black Tentacles. Which is obviously quite fantastic, but doesn’t cover our massive reliance on Hex. 

Resilient helps out more actively. Simply speaking, our preferred methodology for Resilient is +1 Constitution for Constitution Saving Throws. That provides a variably +2 to +7 to Constitution checks over the course of your career (depending on level and if it provides enough Constitution to improve your modifier). This can drastically improve your chance to survive Concentration checks when taking damage.

War Caster is another valid option, more valid if you have even Constitution to start with. Advantage is usually around a +5 in terms of average improvement to dice rolls, so it ends up providing around the same bonus. And it lets you attack of opportunity with Banish. Which is fun.

Best Multiclass Options for Fathomless Warlock

The Fathomless Warlock can benefit quite a lot from Charisma-based classes that aren’t very reliant on bonus actions.

Paladin

The Paladin offers a Fathomless Warlock quite a few benefits. We’d recommend starting Paladin for this theoretical build, so you can begin play with Heavy armor proficiency. However, even if you multiclass into it, Medium Armor and Shields are good enough. Then, you’re getting great actions like Lay on Hands to work with.

At level 2, though, you’re getting the meat-and-potatoes of the Paladin build. A Fighting Style is very nice for you, granting you usually a +1 to AC. But, if you wanna enter the fray with your Tentacle in tow, that’s definitely an option. Divine Smite is where you’re going next, allowing you to utilize spell slots for pure damage.

With this build in mind, a Pact of the Blade Warlock is a great option. You don’t need to invest too much into Paladin to make this build work, but Paladins with Oaths that don’t use bonus actions, like Oath of the Ancients or Oath of Glory, can be handy.

Sorcerer

The Sorcerer lets you run a Coffeelock style build. By turning Warlock spells into sorcery points, you can fuel future sorcery spell slots or metamagic of your choosing. The Tentacle of the Deep is a great bonus action even partially upgraded, so you can invest some time into a Sorcerer of your choice. 

Usually, a Divine Soul or Aberrant Mind sorcerer works best for this style of build. Their flexible spell slots and alternative ways to use sorcery points are quite useful. 

Best Backgrounds for Fathomless Warlock

While you should focus more on character flavor than mechanical power for backgrounds, there are some that a Fathomless Warlock finds particularly useful.

Guild Artisan

The Guild Artisan background, from the Player’s Handbook, is actually pretty good for a Warlock. It starts you with Insight and Persuasion. Persuasion is what Warlock needs to round out its Charisma-based talking skills, so that’s a win on the spot. You get Artisan’s Tools, which are about as strong as your DM lets them be, and a language. A bonus language for the party talker is very nice, especially before you can access Tongues.

You equipment is rather astounding, since you get 15 gold at the starting line. Your Feature is also pretty good, as your guild will actively aid you in many different ways. However, it comes at a cost of 5 gp per month. Depending on how your DM works with your guild, you might be able to avoid paying immediately… But, expect debts to accrue if handled poorly. A strong ability, but a dangerous one.

Investigator

The Investigator, from Van Richten’s Guide to Ravenloft, is one of the more traditionally stronger backgrounds in the game. Insight, Investigation, and Perception are all very strong options, though we’d recommend Insight and Perception to leave you with two Charisma-based abilities. Disguise Kit and Thieves’ Tools proficiencies are both very strong, even if you don’t plan on being the party’s Rogue/Bard.

And that’s not to mention your gear and feature… Neither of which are fantastic. You get a bunch of fluff and 10gp from your equipment, and your feature is that your reputation as an investigator proceeds you. That means you’ll usually gain access to items faster, but locals might know about you and find you dangerous. It’s intentionally worded to be a double-edged sword, so be mindful of your legacy.

Urban Bounty Hunter

The Urban Bounty Hunter, from the Sword Coast Adventurer’s Guide, is a very strong face-based background. You get two skills (we recommend Persuasion and either Insight or Stealth) that can help you effectively talk and walk. Then, you get two tools. Thieves’ Tools are great, but the gaming set or musical instrument is a bit of a coin flip. We’d recommend the musical instrument, since that’s more likely to solve a savage beast than a game of craps.

You basically get nothing but 20 gp for your gear, which is more than fine by me! That’s money you can invest into your first Thieves’ Tools or a stay at an inn. Then, Ear to the Ground gives you contacts in the shady side of the world. Nice for almost any city, since you’re guaranteed a contact in most of them. Though, that contact could be a bit tricky to always have safe access to.

FAQ for the Fathomless Warlock

What book is the Fathomless Warlock in?

The Fathomless Warlock is on page 72 of Tasha’s Cauldron of Everything. It is the first Warlock archetype of the book, four pages after the Clockwork Soul origin of the Sorcerer.

Why is the Fathomless Warlock good?

The Fathomless Warlock’s strength comes from the Tentacle of the Deeps, a fantastic bonus action that can deal damage, proc your Hex an additional time per turn, and provide solid enough crowd control for allies. However, it is very important to use it carefully, especially early on, since you only get your proficiency modifier in uses per day.

Can Tentacle of the Deep get advantage?

Because the Tentacle of the Deeps is not given its own statistic block or its own health bar, the Tentacle is basically an extension of yourself. When you have advantage on the attack, it has advantage on the attack. But, it itself cannot be used for alternative actions like flanking, or anything that it doesn’t innately have as a tentacle.

Example Fathomless Warlock Build

Before we can get started on our Fathomless Warlock, we first need to discuss any and all limitations that our DM is putting on our campaign. Unfortunately for us, our DM is quite ruthless!

• No multiclassing. We’re getting to Warlock 20. That makes some sense, as some DMs would rather not deal with characters that have so many in-built rules.
• No lineage rules. We can’t use the Custom Lineage, nor can we use any alternate ability scores for races. This locks us out of several races, including the Owlkin, which is unfortunate.
• Any official book for race and background. Even without the lineage rules, we still get quite a few options that we can pull in. This’ll also let us use Urban Bounty Hunter, which is very nice.
• Standard Array. That means we assign the numbers 15, 14, 13, 12, 10, and 8 to our ability scores, rather than rolling or point buy. While annoying, it still lets us get the 17, 14, 14 ability score spread that I love for Warlocks.
• Standard Equipment. A little annoying, but the Warlock isn’t exactly desperate for gear (outside of the Hexblade). The Fathomless can do more than fine with generic gear until you can find more.
• Our party is a Paladin, Cleric, and Artificer. This leaves us with kinda limited options. But, it means we are probably going to be the party’s area-of-effect damage dealer. Fathomless does that quite well, thankfully.

Alright, with these requirements, we have a good idea of what our build entails. We’ll become a Levistus Tiefling with an Urban Bounty Hunter background. For the sake of our backstory, we were hunting angels before we had a change of heart.

Since our equipment doesn’t really matter, we just need to talk a little bit about our background and class proficiencies. The Urban Bounty Hunter will start us with Insight, Persuasion, Lute proficiency, and Thieves’ Tools proficiency. That’s a vast number of impactful skills for our Warlock, and will greatly improve our Deception and Intimidation proficiencies. We can now talk to anybody anyway we’d like.

Our party is a bit wonky. We’ll be working with Thieves’ Tools to try and unlock doors for us, even if our Dexterity isn’t going to be too high. We are legitimately quite locked-in to 14 and 14 for DEX and CON.

5E Fathomless Warlock Build

Race: Levistus Tiefling Ability Scores: STR 8, DEX 14, CON 14 (13 + 1), INT 10, WIS 12, CHA 17 (15 + 2) Proficiencies: Deception, Intimidation, Insight, Persuasion, Lute, Thieves’ Tools Starting Equipment: Light Crossbow, 20 Bolts, Component Pouch, Dungeoneer’s Kit, Leather Armor, Quarterstaff, Two Daggers, Angel Hunter’s Clothing, 20 gp Languages: Common, Infernal

Level	Class Benefits	Choices
1	-Pact Magic -Expanded Spell List -Tentacle of the Deeps -Gift of the Sea	We hope you’re not quite done with making choices! We’ve got a few more for you.  Cantrips. Our cantrip choices are basically picked for us. Eldritch Blast is way too strong for any Warlock to avoid. Prestidigitation is our choice for utility cantrip. A bit of a shame, since we have a few utility cantrips that we’re desperate for, but it gives us a handful of useful abilities. Better have a handful of abilities before we get slightly more standard cantrips.  Spells. We also only start with two spells, so we better make them count. Hex will do for damage, and Sleep will do to take out dangerous foes for a time before we can deal with them. Our Tentacle with Hex lets us deal 1d10+1d8+2d6 damage in a round to a single target. We have… A lot of burst, actually. Sleep can absolutely murder low-health targets, so we’ll use it while we can.
2	-Eldritch Invocations	Another two-er! Let’s consider what options we have here.  Spells. In terms of magic, we have a few magical abilities to select. The Warlock doesn’t get too many winners in the 1st level spell slot selection, so we’ll scoop up Hellish Rebuke for now. Definitely throwing it away later, but it’s 11 damage on average as a reaction to simply getting hit. Up to 6d10 at fifth level spells, which is no joke.  Invocations. We start with two additional invocations. For our sake, we’ll scoop up Agonizing Blast to deal additional damage with our Eldritch Blast. That’s way too much damage to leave on the table. Then, we’ll get Armor of Shadows to compensate for our mediocre AC.
3	-Pact Boon -2nd Level Pact Magic	Now to pick our Pact. Hoo boy…  Pact. For our Fathomless Warlock, we’ll actually end up getting Pact of the Chain. Imps are going to be very useful in this party. We need a scout, and Imps deal great damage at this point. We’ll also unlock later invocations that help our utility quite a lot.  Spells. We’ve made it to the 2nd level, and I really like Shatter for us. It’s not exactly the best area-of-effect damage in the game, but it lets us deal with small mooks without needing to spend too many turns. Our party is poor at dealing with small monsters, so having even the most basic option is good.
4	-Ability Score Improvement	The Warlock is one of the few classes in the game that has to pick so many things every level. Which is absolutely great, but does mean we have to think quite hard here.  Feat. We’re going to get Fey Touched here. Misty Step is a crazy strong spell that we’d have to dig for otherwise. For our 1st level spell, we’ll end up grabbing Command. Command can hit 5 creatures at maximum strength which can slow down a ton of monsters. This gets us to 18 Charisma too, which is great.  Cantrip. We get another cantrip at this level. That’s good, since it lets us get Mage Hand, one of the strongest utility cantrips in the game.  Spells. Now that we have a damage option, let’s get a supportive one. Hold Person uses our Concentration, but it also forces a humanoid monster into a save-or-die situation. How can we say no to that? We’ll also replace Sleep at this point, since it no longer hits as many targets as we’d like. Darkness will be a decent replacement.
5	-3rd Level Pact Magic	Another level. Some more magic and another invocation.  Spell. We’ll scoop up Lightning Bolt at this level, which does quite a lot of damage compared to our 4d8 Shatter. Depending on the room size, Shatter still has some utility, so let’s keep it around for now. With that in mind, I’m fairly comfortable with all spells that we have.  Invocation. Since we chose Chain, we’re somewhat obligated to grab Investment of the Chain Master. It makes our Familiar much better at its job in a ton of different ways, including giving us a very solid bonus action.
6	-Oceanic Soul -Guardian Coil	This is the first point that we only need to pick a single thing. Let’s go! Spell. We’ll honestly grab Tongues at this level. We only really know how to talk to demons and Common speakers. Let’s cover our bases for now until we can either find magical items or some other way to translate.
7	-4th Level Pact Magic	Alright, back to our standard.  Spells. Summon Elemental is a good spell, even if it is just bringing out Water Elementals. This can let us cover our bases in the case of a multi-faceted combat scenario. A good use of Concentration, though our spell list is clearly full of those. Thank goodness for Lightning Bolt…  Invocations. Eldritch Mind is great. We just talked about how many concentration spells we have online at a time. So, now we can have advantage on saving in those cases. At this point, we may be reaching Light Armors that can give us 13 AC. If that’s the case, we can tag out our Armor of Shadows for something else. For our build, Devil’s Sight is a nice way to abuse Darkness for the better.
8	-Ability Score Improvement	Things are going to be a bit weird this level. Feat. We’ll just take a +2 to Charisma at this point. 20 Charisma is exactly where we want to be, and we got here right on time (for any race other than Custom Lineage, at least). We’ve maxed out our natural spell attack roll and save DC. Now, let’s take a look at our spell lists.  Cantrips. Eldritch Blast, Mage Hand, Prestidigitation, Ray of Frost Spells. Command, Hellish Rebuke, Hex, Darkness, Hold Person, Misty Step, Shatter, Lightning Bolt, Tongues, Summon Elemental (Water Only)  Not bad! We have spells for several different situations, including great Concentration options. Shatter is likely looking slightly underwhelming at this point, a mere 5d8  (22.5) compared to 9d6 (31.5). So, let’s plan on tagging that out.  Spells. We’re going to learn Raulothim’s Psychic Lance and Counterspell at this level, replacing Shatter. This gives us a great crowd control option and one of the better situational reactions in the game.
9	-5th Level Pact Magic	Another spell and invocation level. Nice and easy.  Spells. We get 5th level spells here, so we’ll be taking the time to finally get rid of Hellish Rebuke. Synaptic Static is a wonderfully strong area-of-effect spell without concentration that also debuffs targets. And targets Intelligence! Hold Monster might not seem strong compared to a Hold Person that targets 4, but being able to hit any monster is well worth it.  Invocation. We have some quite strong invocations available to us at level 9, though none are strictly overpowered in any particular way. We like to actually go back and scoop up Sculptor of Flesh, since Polymorph is an insanely strong spell that you should consider casting daily.
10	-Grasping Tentacles	A weird level for our Warlock.  Cantrips. For emergency situations, Blade Ward might keep us alive for an extra turn while we try to bonus action our way to victory. The Warlock doesn’t get too many utility cantrips, so this’ll work as “utility.” Spells. We automatically learn Evard’s Black Tentacles, so that’s going up there. Evard’s will also be a huge spell for us to consistently cast. Since it’s very difficult to break our Concentration on it, we just want to keep it for locking down doors.
11	-Mystic Arcanum (6th Level)	At this point, we start getting our Mystic Arcanum. These should be spells that we plan on casting every single day, so make sure they’re catchy! Or at least useful enough in standard fights and against bosses.  Mystic Arcanum. We’ve always liked Scatter for a Warlock’s level 6 spell. It offers a lot of utility as an emergency get-off-me tool, a way to position the whole party before a fight, or a way to force a ton of enemies into a bad situation. Mobility is key, and this spell provides quite a lot of it.  Spell. For our regular spell list, we’re actually pretty good on spells we want to cast every day. But, there’s one we haven’t considered yet that we can use for combat control. Bigby’s Hand is a solid, multifaceted spell with a lot of combat utility. Plus, it’s a tentacle. Which is fun.
12	-Ability Score Improvement	Another somewhat busy level for us.  Feat. The Inspiring Leader feat will grant us exactly what we’ve been lacking for a few levels now; realistic durability. Our Cleric’s probably not been too happy with us, since we have pathetic AC and not great health. Now, we can give everyone, including ourselves, 17 extra health whenever we rest. That should be enough incentive for even our Artificer to like short rests.  Invocation. Our Invocations are looking solid enough, and 12 doesn’t really unlock many particularly explosive ones. For that reason, let’s take Repelling Blast to get people away from us.
13	-Mystic Arcanum (7th Level)	More magic? More problems.  Mystic Arcanum. This is usually the level you take Crown of Stars to have a consistent bonus action. We have quite a few solid bonus actions, so we’ll instead take Forcecage, a spell that can completely end some encounters by itself.  Spell. Danse Macabre is a largely potent spell that provides you with quite a few incredibly powerful and damaging summons. While we have great concentration actions to begin with, this’ll let us cover doors or mob someone to death.
14	-Fathomless Plunge	At long last, we get a breather! Take the time to look at your spell list and make sure you have options that you either cast consistently or like to cast occasionally. While I like the look of our spell list on the surface, spells like Hold Person can very quickly lose any relevance in specific campaigns.
15	-Mystic Arcanum (8th Level)	Alright, hope you liked your break. You have three choices this time.  Mystic Arcanum. We are such big fans of Power Word Stun. As long as a boss gets low enough, you can guarantee put them out of a full turn. Depending on turn order, this spell is insane, and can quickly turn the fight into a beatdown. No save on a stun is wild!  Spell. Let’s consider Teleportation Circle as a situational spell that we can use to get back home every now and then. We certainly don’t want any of the Concentration options at this point.Invocation. We have quite a few 15 level invocations that are fairly interesting. Chains of Carceri is our choice, as long as we’re fighting the types of monsters that are listed; celestials, elementals, or fiends. Those are fairly common late game enemies, so it’s like we get a free Hold Monster every day. Good enough for me!
16	-Ability Score Improvement	Another ability score improvement. Our goal for this one is going to be getting two feats with a +1 to Constitution to get that to an even number. This’ll let us give an Amulet of Health to another ally, rather than horde it for ourselves. Our first +1 Constitution feat is Chef. While we give a handy amount of temporary hitpoints, giving our Paladin and Cleric more to work with between each short rest will be good. 6 Temporary Hitpoints is a reasonable bonus action, if not overpowered during a fight. And it’s not like you can short rest after every single fight, realistically.
17	-Mystic Arcanum (9th Level)	Our last Mystic Arcanum, another spell to learn.  Mystic Arcanum. Power Word Kill is usually our choice, but Psychic Scream is our choice for this build. We’re our party’s Area of Effect character, and this area of effect can stun everyone. This’ll be useful for lategame hordes that you’d rather stun out of their turn.  Spell. Sure, we’ll scoop up Dispel Magic. It’s something. Sometimes you need to just turn someone’s spell off in order to save an ally.
18		Despite the fact that our class doesn’t get anything baseline here, we do still get an Invocation.  Invocation. Visions of Distant Realms is an okay scouting feat that goes well with our weaker Familiar. This should be a slot reserved for a heavily campaign-based, endgame invocation. Witch’s Sight, for instance, might let you crack down on a particularly nasty shapechanger.
19	-Ability Score Improvement	Alright, final spell and final feat!  Feat. Our last +1 to Constitution will be… Moderately Armored. Not exactly shocking. Medium Armor and a Shield will give us quite a bit of extra AC to survive into the endgame with. While we were getting hit often, our frontline was keeping us largely safe. When you get to CR 25 enemies, all bets are off. Get yourself some defense!  Spells. As for what spell you get this level, you’re once again going for weird utility magic that you don’t cast all the time. Gaseous Form will work fine to let us sneak out of danger.
20	-Eldritch Master	We’ve made it to the finish line. One last look at our spells.  Cantrips. Blade Ward, Eldritch Blast, Mage Hand, Prestidigitation, Ray of Frost Spells. Command, Hex, Darkness, Hold Person, Misty Step, Counterspell, Dispel Magic, Gaseous Form, Lightning Bolt, Tongues, Evard’s Black Tentacles, Raulothim’s Psychic Lance, Summon Elemental (Water Only), Bigby’s Hand, Danse Macabre, Hold Monster, Synaptic Static, Teleportation Circle Mystic Arcanum. Scatter, Forcecage, Power Word Stun, Psychic Scream  Very happy all-around. We’ve got good, random options for different spell levels, strong once-per-day casts, and even a bunch of utility from our spell list. It’s solid, with even the lower level spells playing a big role. But, feel free to tweak it for your campaign. Spells like Galder’s Speedy Courier can do wonders in, for example, oceanic campaigns.

Conclusion – Our Take on the Fathomless Patron

The Fathomless is a really interesting for the 5E Warlock, and is useful in and out of the water. It’s best in an underwater-based campaign, but brings enough combat tools and environmental control to make a difference wherever you’d like. If you want to play a Warlock that restricts enemy’s options, or are trying out a fully Underwater campaign, this is the choice for you!