Fathomless Warlock 5E Guide | Warlock Subclass from Tasha’s Cauldron

fathomless warlock 5e

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 class 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.

Fathomless Expanded Spells
  • 1st Level – Create or Destroy Water, Thunderwave
  • 3rd Level – Gust of Wind, Silence
  • 5th Level – Lightning Bolt, Sleet Storm
  • 7th Level – Control Water, Summon Elemental (Water Only)
  • 9th Level – Bigby’s Hand (tentacle-shaped), Cone of Cold

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

fathomless warlock 5eThis 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 Tome

The Book of Shadows tends to be the safe choice. If you want to be good in the late game, you pick this pact. 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!


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!

Conclusion – Our Take on the Fathomless Patron

The Fathomless is really interesting, 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!

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