A Complete Guide To Playing A Fathomless Warlock In 5e D&D

The tentacle of ghostly water made an odd gurgling sound before cracking the goblin sharply across the brow, sending it rolling to the Warlock’s feet. “Ah, I see you’ve met my little pet,” the Warlock said, energy gathering in her palm as she looked down. “Now the real fun begins.”

Table of Contents

Ahoy! Today we’re exploring the Fathomless, a Warlock subclass known for lurking in coral beds, sleeping with the fishes, and occasionally siphoning power off of underwater nightmare creatures.

Fathomless is a defensive class, stacking tricks up their sleeves until the perfect opportunity pops up to unleash magic harsher than a tropical storm. Your power comes from patrons who are deeply tied to the sea and channel the magical potential of the salty depths.

So whether you’re a shipwrecked pirate or just a mermaid looking to enter their goth phase, let’s take a look at the Fathomless Warlock!

Using This Guide

Everything in this guide has an emoji, ranking how useful a given ability or feature is to playing this class effectively.

✅ — An absolutely crucial feature. Often forms the backbone of a class’ look and feel. Will provide some bonus that can’t be found through other means.

🆗 — A solid feature that does its job well. Not game-breaking, and certainly replaceable, but a strong choice that shores up some weakness.

⚠️ — A debatable choice. Could work for a specific build, but otherwise is either a wasted opportunity or is just weaker than other alternatives.

⛔ — Outright bad and detrimental. This weighs down the class and just takes up space on the character sheet. A weakness you will have to accommodate for.

These rankings are meant to help you create an optimized class build but remember—DnD isn’t a game where you need to win to have fun. Weaker but flavorful builds also have their place and can make for fulfilling characters.

How To Play A Fathomless Warlock

Fathomless likes acting as a conductor, steering combat and commanding companions to outmaneuver opponents. You will fall back on eldritch blast often, but when you cast a big levelled spell, it will be to upset the battlefield and block incoming enemies (Controlling). You are also the party’s backbone, keeping them on their feet and throwing around a few buff spells (Support).

Leave the damage dealing to your other party members except when you don’t have anything better to do. You’ll need to pick the right moment for a leveled spell, but you can fill your time using cantrips and possibly your Pact Boon to get everything in place, so that your big spell has the maximum effect.

Being An Effective Support

Your main support option is your Tentacle of the Deeps. At Level 6 it can start soaking up damage, so keep it near your allies and let it save lives. With its long range, the tentacle also defends your party’s flanks if placed cleverly.

Other than the tentacle, you can grab a few spells to defend your friends. Many of these—the ever-handy magic circle in particular—reward you for keeping your party grouped up. Stick close and between your spells and the tentacle, nothing should be able to hit too hard. Look to keep yourself alive with good defensive options to avoid going down too often and becoming a burden.

Being An Effective Controller

If your party is grouped up, you are vulnerable to one big attack wiping you all out at once. Your control spells will need to prevent this. Debuff strong monsters or cripple spellcasters to avoid any big attacks and look for ways to block line-of-sight, making those attacks more difficult to land.

You can also put up obstacles like walls or difficult terrain that weaken melee enemies. This gives you time to pick them off with eldritch blast and allows your party to catch their breath and strategize.

Fathomless Subclass Features

Your most impactful subclass feature is a spectral tentacle that helps you in combat and gets progressively better as you level up. This is complemented by several abilities which strengthen you while in the water, making you a threat both on land and in the sea.

🆗 Expanded Spell List

1st Level

⛔ Create or Destroy Water

Warlock doesn’t have the spell slots to waste on meager utility effects like this, and there just aren’t that many situations where you need a couple dozen gallons of water.

⚠️ Thunderwave

Reliable damage that scales when cast at higher levels. Sadly, the short range means you have to put yourself in harm’s way to cast it, or else already be flanked by enemies.

2nd Level

⚠️ Gust of Wind

A very narrow area-of-effect means most enemies can sidestep the wind tunnel. Use this to take out one target, but don’t count on it being useful for consecutive turns.

🆗 Silence

Very good against spellcasters, moderately bothersome against everyone else. Can effectively shut down a fight against spellcasting enemies, but be careful not to hit yourself too.

3rd Level

🆗 Lightning Bolt

While the damage is comparable to fireball, the area-of-effect is not. A single line will hit far fewer targets. Still, it is 100 feet long, which can snipe enemies, even those in mid-air, so that’s something.

✅ Sleet Storm

This is a phenomenal control spell for being such a low-level. It blocks line-of-sight in a large area and makes it difficult terrain. Very good for sealing off large doorways or corridors and preventing enemies from pursuing your party.

4th Level

⚠️ Control Water

This certainly gets better if you’re in an aquatic campaign, when you can essentially wield water like the Force. Otherwise, you won’t find a large body of water often enough to make this good.

🆗 Summon Elemental (water only)

A big, sturdy water elemental is a nice companion in combat (though don’t rely on its less-than-impressive offensive capabilities). This spell is somewhat weaker than its old equivalent conjure elemental, but the elemental no longer goes rogue and attacks you if your concentration is broken, which is nice.

5th Level

✅ Bigby’s Hand

Upsized mage hand. The hand has some impressive stats and hits quite hard, and that’s only if you want to use it to fight. There are a multitude of clever ways to use this, like launching yourself through the air or opening holes in walls.

✅ Cone of Cold

Simply a very good balance of high damage and large range. An efficient way to kill things, and will probably be your go-to area-of-effect damage spell as Fathomless.

⚔ Player Tip: Learning Hydromancy ⚔

Hoping for more water-based spells for your water-Warlock? Check out this complete list of spells that manipulate or summon water from DNDBeyond, though be warned you might need to multiclass to obtain some of them.

🆗 Tentacle of the Deeps

Warlock has a chronic bonus action issue, in that they never get to use it. Your magical tentacle pet fills that role, as you suddenly get to deliver extra damage with your bonus action! On top of eldritch blast, that brings you to a total of 4d10 + 2d8 damage per turn at max level. This is an excellent way to crank your damage output to eleven, and the slow effect helps you control the battlefield, especially when you want to lock an enemy in place for one of your area-of-effect spells. Recharging on a short rest means you’ll be able to drop one of these tentacles in essentially every battle.

⚠️ Gift of the Sea

It’s fine. Most useful in an oceanbound or underwater campaign. And if you’re playing in one of those, look to gain these crucial bonuses from your race, rather than your subclass.

🆗 Oceanic Soul

A damage resistance never hurts, and cold damage is reasonably common. Talking underwater might seem good for a high-Charisma class, but it’s so niche you’ll probably never get to use it.

✅ Guardian Coil

This turns your tentacle from “neat” to “amazing”. You now get to use up your action, bonus action, and reaction all for useful effects. Reducing damage is always good, and a 2d8 reduction is nothing to scoff at. Just be sure you keep your tentacle near enemies in a melee brawl rather than across the room, so it can protect your allies.

✅ Grasping Tentacles

Evard’s Black Tentacles is an excellent control spell, dealing damage while also slowing enemies and restraining a handful of them. Getting free health is cool, but never breaking concentration until you drop the spell is bonkers good. As is getting to cast it once a day without using a spell slot. This ability removes many downsides of Warlock’s spellcasting, and even though that’s only for one spell, it’s a damn good spell.

⚠️ Fathomless Plunge

So to teleport the party, you need to have already seen the target location, and even then it can only be up to one mile away. That’s not much. This is really more of a get-out-of-jail-free card that lets you run away from a scary fight. While that’s not a bad ability, there are a few too many caveats for it to qualify as “good”.

Read our full guide on Warlock spells here.

The Perfect Fathomless Build

Your build should focus on keeping yourself alive so that your allies can rely on you to help them. Better armor, protective spells, and high defensive abilities are key. Once that’s settled, you’re an impressively well-rounded subclass, so look for ways to shore up any shortcomings and you’ll never find yourself in a situation you don’t have an answer to.

Ability Scores

⛔ Strength

Dump stat.

🆗 Dexterity

A high Dexterity save and a high AC are both good defensively, so when you have ability score increases to spare, give this some love.

🆗 Constitution

You will want this to be quite high. Aside from more health which is good for a ranged spellcaster such as yourself, a good Constitution save keeps your powerful concentration spells running.

⛔ Intelligence

Dump stat.

⛔ Wisdom

Dump stat.

✅ Charisma

Fathomless likes having a lot of effects running at once via their subclass features and spellcasting, and Charisma is the ability score you use for the latter. Pretty obvious—this is your most important ability. Particularly for eldritch blast shots and your spell save DC, this needs to be as high as possible.


Eladrin, Half-Elf, Satyr, Verdan, Warforged, and Yuan-Ti Pureblood are all excellent choices, granting a Charisma bonus and strong defensive abilities to keep you stubbornly alive.

🆗 Changeling, Tabaxi, and Variant Human are all flexible races with abilities that complement those which Fathomless supplies. Tiefling is also on this tier, because while it makes a very good Warlock, it can be hard to reconcile Tiefling’s flavor with the Fathomless subclass.

⚠️ Simic Hybrid and Triton. Why are two of the most aquatically-capable races ranked so low? Well, Fathomless already turns you into an amphibian, so it’s actually a waste to get the same features from your race.


🆗 Elemental Adept

Your subclass spell list dips its toes into several damage types (cold, lightning, and thunder), each of which could be chosen for this feat. If you are drawn to a build focused on one of these damage types, this feat powers your attacks up by bypassing resistances.

✅ Moderately Armored

You can’t help your party if you’re dead. And your tentacle will disappear if you drop, too. Staying on your feet is crucial for Fathomless, and good armor helps immensely. Just be aware that if your Dexterity is high enough, you may be better off with light armor instead.

✅ War Caster

You are a spellcaster and therefore concentration checks are your mortal enemy. This feat makes them much easier and helps keep your spells running, saving your precious spell slots that might be wasted on recasting spells.


✅ Agonizing Blast

Better eldritch blast is always a necessity for Warlock.

⚠️ Beast Speech

Perhaps a strange choice, but if you’re going to be swimming around a lot, who doesn’t want to talk to fish?

🆗 Fiendish Vigor

False life is often that extra little bit of health you need to stay on your feet. Perfect for a subclass that wants a strong defense.

⚠️ Gift of the Depths

You don’t need the aquatic abilities yourself (your subclass features see to that), but casting water breathing on your party once a day might just keep them from drowning.

Read our full guide on Warlock Eldritch invocations here.

Warlock Pact Boons

Acting as a blend of control and support, Pact of the Talisman is a strong choice. The Protection of the Talisman Invocation helps keep you alive, survivability you desperately need, or you can pass it on to a friend! Similarly, the Bond of the Talisman Invocation can blink you or an ally away from danger and helps make sure you’re always ideally positioned on the battlefield.

Pact of the Chain is also a valid choice if you are really enjoying your Tentacle of the Deeps and would love a second one. You absolutely need to take the Investment of the Chain Master Invocation, but if you do, your familiar and your tentacle can cover both of your party’s flanks and give you more flexibility with how to use your bonus action.

Pact of the Tome nevers hurts for some extra spells, but doesn’t synergize well with Fathomless. The only exception is the flavorful cantrip shape water, which you can read all about in this guide from RPGBOT. Pact of the Blade is alright if you want to join your tentacle in combat where it can soak up damage that might otherwise kill you, but you really aren’t strong enough in melee for this strategy to be particularly effective.



✅ Eldritch Blast

Forever-and-always the best Warlock spell. This will be your default attack for your entire time as a Warlock.

1st Level

✅ Armor of Agathys

A strong defensive measure which can also hurt your attackers, making it very good against hordes of weak enemies. Excellent at keeping you alive, which is helpful.

2nd Level

🆗 Darkness

Good for blocking line-of-sight and making enemies close the distance to within range of your tentacle.

✅ Mirror Image

Just stupidly good. Even if something hits you, there’s a 75% chance it misses anyways. Lasts for a long time and blocks a lot of hits while doing so, making this an incredibly efficient use of your spell slot.

3rd Level

✅ Hunger of Hadar

One of the best control spells you’ll have access to. The penalties are so intense that whatever gets stuck in the spell’s area-of-effect will be staying there for the majority of a fight, and will be greatly weakened.

🆗 Intellect Fortress

A nice buff to slap on an ally and make them much better at resisting common spells.

✅ Magic Circle

This is just such a versatile protection spell. Keep your party grouped up in the circle and defend them with your tentacle. That combo alone can win fights with how good the buffs from this spell can be. Also capable of trapping a couple of scary enemies.

4th Level

🆗 Shadow of Moil

Another powerful defensive spell which excels at protecting you from crowds. Kills small creatures outright, and the darkness should obstruct you and make ranged attacks hit less.

5th Level

🆗 Synaptic Static

Great multi-target damage, but debuffing a save is particularly nice. If anything survives your attack, it is easier to hit with successive spells, including simply casting this one again.

🆗 Wall Of Light

When you need a big flashy control spell, look here. A giant obstacle getting conjured tends to disrupt the battlefield, and blocking off a group of enemies with a big wall is endlessly useful.

6th Level

🆗 Mental Prison

Loads of damage if the effect is broken, which is great. Otherwise, you leave the biggest baddest enemy shaking in their boots, unable to fight and wide-open to attacks.

🆗 Tasha’s Otherworldly Guise

Ignore the weapon attacks, and just use this spell for the defensive buffs. You just get so many nice bonuses that most attacks against you are weakened in some way.

7th Level

✅ Forcecage

Perfect for tall monsters and wide crowds alike! Tag your choice of enemies out, and they can no longer fight your party while your concentration holds. Such a strong control spell that makes it easy to pick off clumps of enemies one at a time.

8th Level

🆗 Dominate Monster

Eliminating a threat? Great. Gaining an ally? Lovely. Doing both at once with a single spell like dominate monster? Phenomenal.

🆗 Feeblemind

Your build is countered best by enemy spellcasters, so turn their brains to goo and make sure they can never hurt you again!

✅ Maddening Darkness

An incredible control spell to create an insurmountable obstacle. Seriously, nothing is getting through this alive. Try dropping it in a closed chamber and see how many monsters walk it off. Spoilers: none.

9th Level

✅ Foresight

So. Many. Buffs. Cast this spell as much as possible. Do it. This is the ultimate support spell, and turns a friend into an unstoppable force of perfect dice rolls.


With your light armor proficiency, studded leather offers the best protection. That said, it is recommended that Fathomless finds medium armor proficiency ASAP.

You won’t be much of a fighter, but if you want a simple weapon on-hand, look for a greatclub, handaxe, javelin, mace, quarterstaff, or spear. If ranged damage is more your style, a light crossbow is your best choice.

If your campaign happens to be oceanbound (which if you’re picking Fathomless, there’s a good chance it is), a set of navigator’s tools can be a lifesaver.

Though it excels in water, Fathomless can be lots of fun even for a landlubber. If this guide helped you see what’s great about this subclass, share it with your friends! And leave a comment below sharing your wackiest ideas for an aquatic or oceanbound campaign.

Leave a Comment