Fathomless Warlock

Published on April 29, 2024

Drag your enemies down with the Fathomless Warlock, a controller-mage who entangles foes to turn the tide of battle. Read our DnD 5e subclass guide.

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Quick Links
Fathomless Warlock Overview
Fathomless Warlock Subclass Features
How to Play an Fathomless Warlock
Ability Scores for Fathomless Warlocks
Fathomless Warlock Races
Fathomless Warlock Feats
Fathomless Warlock 5e Build Example

Fathomless Warlock Overview

“I cannot think of the deep sea without shuddering at the nameless things that may at this very moment be crawling and floundering on its slimy bed…”

—H.P. Lovecraft

Have you always wanted to yell ‘release the kraken’ at the DnD table? Just obsessed with tentacles in general? Fear not — the Fathomless Warlock is here to make your dreams of Lovecraftian domination come to life. As a Fathomless, you have made a patron pact with some deep-sea entity like a kraken, a water elemental, or a sea hag.

From a near-drowning to a shipwreck, your encounter with the murky depths has given you the powers to summon the might of the oceans in battle. This warlock subclass will be nightmare fuel for your party members’ thalassophobia, thematically inspired by the nameless things that lurk below.


This subclass comes with a lot of crowd control and disruption effects through your summoned Tentacle of the Deep, along with your signature subclass spells. An optimized build and good spell choices will help keep your enemies where you want them — within the reach of your appendages. Depending on the setting of your campaign, this subclass can also offer unparalleled utility in aquatic environments.


The biggest drawback to this subclass is a disappointing ultimate feature in Fathomless Plunge, which may convince many players to spec into a second option after 10th Level. The subclass also has some decidedly-average expanded spells, so you may find yourself relying on core Warlock spells instead. Fortunately, these options fit well into your control-centric gameplan, so it’s not the end of the world — at least, not for you.

Subclass Guide Rating Scheme

This guide is meant as a deep dive into the Fathomless subclass. For a full overview of the Warlock class, check out our Warlock 5e Guide.

For our class guides, we use the following color rating scheme:

  • Red isn’t going to contribute to the effectiveness of your character build at all.
  • Orange Situationally good, but a below-average option otherwise.
  • Green is a good option.
  • Blue is a great option, you should strongly consider this option for your character.
  • Sky Blue is an amazing option. If you do not take this option, your character would not be optimized.

Fathomless Warlock Spells (Pact Magic)

The otherworldly pact between you and your aquatic patron lets you tap into the might of the ocean deeps. These spells are either utility options or provide thunder/frost-type damage.

  • 1st level
    • Create or Destroy Water: Being able to conjure 10 gallons of water isn’t particularly effective unless you are dying of thirst in a desert.
    • Thunderwave: A fantastic, low-level way to knock opponents back if you find yourself in a sticky situation. Damage isn’t bad either but it targets CON saves.
  • 2nd level
    • Gust of Wind: This spell is usually useless unless you find yourself in a rare situation where you can use it to push multiple enemies off of a cliff.
    • SilenceSilence is a niche spell with a high ceiling. It can be used in stealth scenarios but it’s most powerful usage is if you can target a caster who won’t be able to cast spells requiring a verbal component. Of course, it’s only a 20ft radius so you will either need to be fighting in close quarters or will need to find a way to prevent the caster from moving.
  • 3rd level
    • Lightning Bolt: Does as much damage as fireball but has a less effective AoE because it’s a line rather than a circle.
    • Sleet Storm: Messes with enemy concentration, can extinguish flames, and has the potential to knock enemies prone.
  • 4th level
    • Control Water: A very effective spell, but only if you’re around water.
    • TCoE Summon Elemental: Unfortunately, the water elemental is by far the weakest summon because it has the worst damage resistance and movement buff. That said, it is still effective and scales with higher spell slots.
  • 5th level
    • Bigby’s HandBigby’s Hand truly does it all. It can do turn after turn damage, help you escape dangerous situations, hold down a baddy, and allow you to fly all while increasing your action economy because it only uses a bonus action to command. Warlock’s usually want to keep their bonus action open for hex, but the utility that Bigby’s Hand provides is well worth it.
    • Cone of Cold: Not quite as potent as Fireball or Lightning Bolt for the resource of a 5th-level spell slot and it targets CON saves which are a common proficiency in monsters.

Fathomless Warlock Subclass Features

When playing the Fathomless Warlocks in DnD 5e, you gain access to a number of nautically-themed abilities that will help you control the terrain of the battlefield. You’ll also be more at home in the water with each level, gaining the ability to swim, breathe, and eventually speak in the ocean’s depths.

1st Level

Tentacle of the Deeps

You can magically summon a spectral tentacle that strikes at your foes. 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.

As a bonus action on your turn, you can move the tentacle up to 30 feet and repeat the attack. You can summon the tentacle a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

The basic feature of the Fathomless is a thalassic version of the Spiritual Weapon spell, a summonable tentacle that can attack nearby enemies and can be repositioned with a bonus action. This feature also comes with a couple of big advantages: no concentration required to maintain it, an extended reach, and the ability to slow enemies on hit.

Aside from the delightful mental image of a tentacle wailing on your opponents, this basic feature is a very strong tool in the Fathomless Warlock’s arsenal.

Gift of the Sea

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

Better swimming speed and breathing underwater are nice-to-have abilities, but tend to be pretty useless unless you frequently find your character in aquatic environments.

6th Level

Oceanic Soul

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.

Similar to Gift of the Sea, this is a niche feature that can be beneficial in certain campaign settings or superfluous in others. Cold resistance is something that should help out in at least a few encounters, but talking underwater will rarely be useful.

Guardian Coil

Your Tentacle of the Deeps can defend you and others, interposing itself between them and harm. 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.

Adding a damage reduction ability to your tentacles with a reaction is a solid feature that really incentivizes smart tentacle placement and encourages your party to fight around the tentacles as well. Damage mitigation might not sound like much, but an ounce of prevention is worth a pound of cure.

10th Level

Grasping Tentacles

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.

This feature grants you access to a powerful, crowd-control spell with considerable upside. Being able to cast it the first time without expending a spell slot is great for the spell-starved Warlock. On top of that, getting temporary hit points and not being vulnerable to losing concentration means you’ll be casting this spell whenever it’s available. Tentacles for everyone!

14th Level

Fathomless Plunge

You can magically open temporary conduits to watery destinations. 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.

Once you use this feature, you can’t use it again until you finish a short or long rest.

As an ultimate feature, the Fathomless Plunge is an unfortunately underwhelming capstone to an otherwise awesome subclass. Teleportation can be valuable and this feature can serve as a way to escape unwelcome or potentially campaign-ending encounters. But you won’t be using this ability to win any fights — just to run away — which makes it pretty lackluster for late-game.

How to Play Fathomless Warlock

When playing a Fathomless Warlock, you will have a clearly-defined playstyle that’s required to maximize the utility of your tentacles. That said, you have a number of creative options for playing it out and dragging your enemies down with you.

Controller Mage

The core identity of the Fathomless subclass is a classic controller mage. Between your Tentacle of the Deep summons and later spells like Evard’s Black Tentacles, your goal will be to tie up opponents and keep them locked down for your other party members to deal damage.

There are a number of ways to get the most usage out of your tentacles. One option is to select additional spells that will hamper opponents, either slowing or immobilizing them. The Warlock spell catalogue has a ton of options to help with this, with notable options being Hunger of Hadar, Hypnotizing Pattern, and Hold Person/Monster .

Another great choice — and the one we’re recommending in our build example below — is to tweak the usual Eldritch Blast build by adding invocations like Lance of Lethargy and Repelling Blast. These invocations will add extra slow effects or knockbacks to keep enemies within range of tentacles, so there’s no chance of escape.

Oh Captain, My Captain?

Any warlock is a good candidate for the face of the party, thanks to their high Charisma stat and useful social skills like Deception and Intimidation. The Fathomless is no different in this regard, which can make them a natural leader in the group.

In addition, the Fathomless can serve a useful out of combat role in a number of other ways. This character will be able to deftly navigate the challenges of aquatic environments, if your campaign trends that way. They also have great tools for scouting ahead if they select Pact of the Chain option at 3rd Level and associated invocations.

Melee or Ranged Playstyle

The limitation of any Warlock character is their low number of spell slots. There are ways to work around this with well-chosen feats and the Fathomless has more tools than most subclasses, thanks to its tentacles, but dealing consistent damage is still an issue you’ll have to solve. The question is: ranged or melee?

There is certainly an argument with a Fathomless Warlock to opt into a melee playstyle. This will allow you to stay consistently within range of your tentacles and get maximum use out of their effects, but Warlocks also tend to be fragile characters and there are risks that come with this gameplan.

At the end of the day, we think the Eldritch Blast build is the superior option. This will allow you to stay at range, focus on your spell disruption and kite enemies around the battlefield. Both choices are probably viable with the right support, but a ranged playstyle seems easier to execute, based on our experience.

Ability Scores for Fathomless Warlock

Charisma is the key ability score for Fathomless Warlocks, just like for another other Warlock subclass. You will also want to prioritize Dexterity and Constitution as secondary ability scores to increase your armor class and hit points. Finally, it can be useful to put stats in Wisdom to help with saving throws.

If you’re using point buy, a recommended starting stat distribution could be:

  • Strength: 8 (-1)
  • Dexterity: 14 (+2)
  • Constitution: 14 (+2)
  • Intelligence: 8 (-1)
  • Wisdom: 12 (+1)
  • Charisma: 15 (+2)

If using another ability score generation method, just follow the color coding above.

Fathomless Warlock Races

Any race that makes a good warlock will make a good Fathomless warlock. See our suggestion for warlock races here.

Interestingly enough, taking an aquatic race, like the triton, lizardfolk, or water genasi, is actually disadvantageous for Fathomless Warlocks, despite the thematic synergy. These races get innate swimming speeds and the ability to breath under water, which makes the Fathomless Warlock’s Gift of the Sea redundant.

Fathomless Warlock Feats

Any feat that works for a normal warlock will work for a Fathomless warlock. See our suggestion for warlock feats here.

The Fathomless 5e Build Example

Our build example for the Fathomless Warlock subclass uses a tweaked Eldritch Blast build to increase your character’s overall crowd control, while selecting Pact of the Chain to help you plan for encounters more effectively.


We’ve chosen Half-Drow from the Sword Coast Adventurer’s Guide as the race for our build. This race gives access to an additional pool of spells, which can be useful for Warlock, plus ASIs to Charisma that are beneficial to the class in general. There are a lot of other good race options, which you can check out in our guide to best warlock races.


As a background, Sailor is a great pick for Warlock overall and matches the specific flavour of the Fathomless subclass perfectly. This background gives access to Athletics and Perception, which are both key skills in a number of different situations. Plus it provides an ideal excuse to speak in a pirate accent. Arrr, ahoy matey!

Eldritch Invocations

As discussed above, the Fathomless Warlock works especially well if you opt for the classic Eldritch Blast build and add a few extra invocations. Choose Agonizing Blast at 2nd level for additional damage, then supplement with Lance of Lethargy and Repelling Blast to provide additional crowd control

Both of these invocations are amazing at keeping enemies in range of your tentacles with slow effects or even pushing them back into range. Combined with the tentacle’s existing slow, this will help you kite foes effectively and stay out of range for as long as possible.

There are a number of great invocations to pick up as you gain levels, depending on what Pact you decide to take. In this build, we opted for Pact of the Chain to gain access to invocations like Investiture and Voice of the Chain Master, as well as the powerful Chains of Carceri at the 15th level. You can view our Eldritch Invocation Guide for more details.

Pact Boon

For the Fathomless, we have chosen Pact of the Chain as our 3rd level Pact Boon. This is one of the two stronger options for pacts. This option allows you to function more effectively as a scout for your party, which means you can plan ahead for engagements and figure out where to position your tentacle and spells most effectively.


For this build, we suggest taking Fey Touched as your first feat at 4th level. This is probably the strongest Warlock feat in the game, so you’ll want to pick it up as soon as possible for a fully optimised build. It works especially well with the Fathomless subclass, as it allows you to reposition closer to your tentacles for additional safety.

After 4th Level, you have more options in terms of what you select for feats. In this example, we’ve suggested taking Divinely Favored at 8th Level to unlock more spells and be able to cast Hex more consistently. There are a number of other options available, which you can check out in our Warlock Feat Guide. You can also always opt for Ability Score Improvements, which we’ve done at the higher levels.

Mystic Arcanum

When choosing your Mystic Arcanum spells, you generally just want to go for the most powerful late-game tools. We recommend checking out our Warlock Spell Guide to see what higher-level spells are the best options for your Mystic Arcanum.

1st level
  • Race: Half-Drow
    • Ability Score Increase: +2 Charisma, +1 Intelligence, +1 Wisdom
    • Darkvision
    • Fey Ancestry
    • Skill Proficiency: Persuasion, Stealth
    • Drow Magic: Dancing Lights, Faerie Fire, Darkness
  • Class: Warlock
    • Armor Proficiency: Light
    • Weapon Proficiency: Simple
    • Skill Proficiencies: Deception, Intimidation
  • Ability Scores (Point Buy)
    • Base: STR 8 (-1) DEX 14 (+2) CON 14 (+2) INT 9 (-1) WIS 13 (+1) CHA 14 (+2)
    • With Improvements: STR 8 (-1) DEX 14 (+2) CON 14 (+2) INT 10 (+0) WIS 14 (+2) CHA 16 (+3)
  • Background: Sailor
    • Skill Proficiencies: Athletics, Perception
  • Otherworldly Patron: Fathomless
    • Expanded Spell List
    • Bonus Cantrips
    • Tentacle of the Deeps
    • Gift of the Sea
2nd level
  • Eldritch Invocation
    • Agonizing Blast
    • Lance of Lethargy OR Repelling Blast
3rd level
  • Pact Boon - Pact of the Chain
    • Imp
  • Eldritch Invocation: Investment of the Chain Master
4th level
  • Feat: Fey Touched
    • +1 Charisma 17 (+3)
    • Hideous Laughter
5th level
  • Eldritch Invocation: Maddening Hex
6th level
  • Fathomless Patron Feature: Oceanic Soul
  • Fathomless Patron Feature: Guardian Coil
7th level
  • Eldritch Invocation: Tomb of Levistus
8th level
  • Feat: Divinely Favored
9th level
  • Eldritch Invocation: Ascendant Step
10th level
  • Fathomless Patron Feature: Grasping Tentacles
11th level
  • Mystic Arcanum (6th level): Mass Suggestion
12th level
  • ASI: +1 Charisma 18 (+4), +1 Constitution 15 (+2)
13th level
  • Mystic Arcanum (7th level): Forcecage
14th level
  • Fathomless Patron Feature: Fathomless Plunge
15th level
  • Mystic Arcanum (8th level): Dominate Monster
  • Eldritch Invocation: Chains of Carceri
  • Eldritch Invocation: Shroud of Shadow
16th level
  • ASI: +2 Charisma 20 (+5)
17th level
  • Mystic Arcanum (9th level): True Polymorph
19th level
  • ASI: +1 Constitution 16 (+3), +1 Dexterity 15 +2
20th level
  • Eldritch Master

Other Warlock Guides

Alex Hamilton

Alex is a content writer for Arcane Eye and a speculative fiction author. His short fiction has appeared in Seize the Press and Daikaijuzine. In his spare time, he enjoys board games, bouldering, and hiking. You can follow him on Twitter @WAHamiltron

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