A Warlock of The Fathomless

The Fathomless Warlock

You have plunged into a pact with the deeps. An entity of the ocean, The Elemental Plane of Water, or another otherworldly sea now allows you to draw on its thalassic power. Is it merely using you to learn about terrestrial realms, or does it want you to open cosmic floodgates and drown the world?
Perhaps you were born into a generational cult that venerates the Fathomless and its spawn. Or you might have been shipwrecked and on the brink of drowning when your patron's grasp offered you a chance at life. Whatever the reason for your pact, the sea and its unknown depths call to you.
Entities of the deep that might empower a warlock include Krakens, Ancient Water Elementals, Godlike hallucinations dreamed into being by Kuo-Toa, Merfolk Demigods, and Sea Hag Covens.
The Fathomless is a servant of some sort of an aquatic creature, and the subclass’s features reflect that theme very effectively. However, much of the subclass is dedicated to functioning in and around water. If your patron sends you onto dry land to do its bidding, big chunks of the subclass dry up (see what I did there) and cease to be useful.
In an aquatic or naval campaign, this is a good subclass. I could see a Fathomless Warlock fighting on the deck of a ship, or facing down tritons and merfolk while raiding a submerged dungeon, but in a typical land-based campaign you’re going to spend a lot of time wishing that you were elsewhere. From the list of current published campaigns, that means Ghosts of Saltmarsh is basically the only one where The Fathomless is a good fit, and considering that there are 8 published campaigns as I’m writing this paragraph (December of 2020), fitting into one of 8 campaigns isn’t a good success rate.

Expanded Spell List

A lot of situational options, and the combat options aren’t great. The spell list gets a little better if you’re underwater frequently, but even then there’s not a lot to be excited about.
The Fathomless lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

Fathomless Warlock Expanded Spells

Spell Level	Spells
1st	Create or Destroy Water, Thunderwave
2nd	Gust of Wind, Silence
3rd	Lightning Bolt, Sleet Storm
4th	Control Water, Summon Elemental
5th	Bigby's Hand (appears as a tentacle), Cone of Cold

Tentacle of the Deep

At 1st level, 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.
At 1st level, this matches the effects of Ray of Frost (at least in terms of damage and the secondary effect. With a 1-minute duration it will get you through a single encounter each time you use it, provided that the encounter doesn’t move too far away from the tentacle to reasonably bring the tentacle back into range. This reduces your reliance on Hex at low levels, allowing you to use your one or two spell slots elsewhere without sacrificing damage output. However, since the daily usage limit is tied to your Proficiency Bonus you won’t be able to use this as often as you could use Hex. Consider the two options complements to each other: use Hex if you just need to Eldritch Blast stuff, and use Tentacle of the Deeps when you need to use Concentration on something other than Hex.

Gift of the Sea

Also at 1st level, you gain a swimming speed of 40 feet, and you can breathe underwater.
Great in an aquatic campaign, but otherwise very situational.

Oceanic Soul

At 6th level, 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.
Damage resistance is great, and in an aquatic campaign the ability to speak to all submerged creatures will make it much easier for you to play a face. However, outside of aquatic campaigns you may find it difficult to convince creatures to talk to you while underwater unless you already share a language.

Guardian Coil

At 6th level, 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.
You likely don’t have any interesting ways to use your Reaction, so using it to reduce damage to yourself or an ally is an excellent addition to your capabilities. With a 1-minute duration on your tentacle, you could reduce up to 10D8 (20D8 at level 10) damage every time you use it, dramatically improving your party’s ability to handle damage without resorting to in-combat healing.

Grasping Tentacles

Starting at 10th level, 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 using 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 Black Tentacles is a good spell. I don’t know if it’s better than Hunger of Hadar, but the added benefits from this feature certainly help. You get to cast this once per day for free, which helps with the Warlock’s tiny pool of spell slots, and any time you cast it (free or otherwise), you get temporary hit points. Being unable to lose Concentration on black tentacles due to taking damage also helps quite a bit in combat, making this an easy go-to option in dangerous situations.

Fathomless Plunge

When you reach 14th level, 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.
With a 1-mile range, this isn’t exactly a long-distance teleportation option. Rather, this is a panic button. When things are going poorly, grab your friends and retreat to a nearby body of water. However, keep in mind this feature has very strict limitations. You can bring 5 other willing creatures, so if your party is bigger than that you’re in trouble. There’s also some debate over whether unconscious allies can be “willing”, so if allies are down you might not be able to save them. The 1-mile range is small, so you need to be very careful to stay near a suitable body of water, which is hard when you’re delving dungeons or wandering the planes. Of course, it’s great if you’re in an ocean because you can just pop out in a different part of the ocean.

1st-level Spells

Two situational options. Thunderwave is fine if you get stuck in melee or need to break a grapple, but I wouldn’t consider it a go-to damage option because getting close enough to hit multiple targets often means getting into melee by choice.

2nd-level Spells

Gust of Wind is very situational, but Silence is a huge problem for enemy spellcasters, especially if your party can keep them inside the area by grappling them, hitting them with a tentacle which applies a speed reduction, or otherwise inhibiting their movement.

3rd-level Spells

Bolt is fine damage, but hitting more than two targets with line effects is hard regardless of the line’s length. Sleet Storm is decent area control but very hard to use effectively, doesn’t scale with spell slot level, and requires Concentration.

4th-level Spells

Control Water is very situational in non-aquatic campaigns, and limiting Summon Elemental to water elementals similarly handicaps the spell unless you’re in an aquatic campaign.

5th-level Spells

Bigby’s Hand is a great, versatile spell. Cone of Cold is decent but unremarkable AOE damage, but if you need area damage you’ll have better results with Hunger of Hadar.

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