Wastrilith

From Mordenkainen Presents: Monsters of the Multiverse (2022):

Found in the waters of the Abyss and other bodies of water contaminated by that plane’s fell influence, wastriliths establish themselves as lords of the deep and rule their dominions with cruelty.

A wastrilith pollutes the waters around it. Its noxious presence even affects nearby sources of water when the demon travels on land. The corrupted water, which contains a measure of the demon’s essence, responds to the wastrilith’s commands—perhaps hardening to prevent foes from escaping or erupting in a surge that drags victims into its reach.

Creatures that ingest water corrupted by a wastrilith risk their very souls. Those who drink the poisonous liquid might wither away until they finally die, or they remain alive only to become thralls of chaos and evil. To represent this defilement, you can use the optional rule on abyssal corruption in chapter 2 of the Dungeon Master’s Guide, causing the poisoned creature to be corrupted.

From the Dungeon Master's Guide (2014):

A non-evil visitor that finishes a long rest in the Abyss must make a DC 10 Charisma saving throw. On a failure, the creature becomes corrupted. Refer to the Abyssal Corruption table to determine the effects of this corruption. You can substitute different corruption effects of your own creation.

After finishing a long rest, a corrupted creature can make a DC 15 Charisma saving throw. On a successful save, the corruption effect ends. A dispel evil and good spell or any magic that removes a curse also ends the effect.

If a corrupted creature doesn't leave the plane within ld4 + 2 days, its alignment changes to chaotic evil. Casting the dispel evil and good spell on the creature restores its original alignment.

Abyssal Corruption:

d10 Result:

1-4: Treachery. The character gains the following flaw: "I can only achieve my goals by making sure that my companions don 't achieve theirs." 

5-7: Bloodlust. The character gains the following flaw: " I enjoy killin g for its own sake, and once I start, it's hard to stop."

8-9: Mad Ambition. The character gains the following flaw: " I am destined to rule the Abyss, and my companions are tools to that end."

10: Demonic Possession. The character is possessed by a demonic entity until freed by dispel evil and good or similar magic. Whenever the possessed character rolls a 1 on an attack roll, ability check, or saving throw, the demon takes control of the character and determines the character's behavior. At the end of each of the possessed character's turns, he or she can make a DC 15 Charisma saving throw. On a success, the character regains control until he or she rolls another 1.

From Planescape Monstrous Compendium Appendix I (1994):

The standoffish wastrilith, or water lords, inhabit watery parts of the Lower Planes (and, rarely, other planes), content in their isolation. Though they have no apparent magical power over other creatures, they intimidate undersea denizens so much that even unintelligent creatures serve them.

Wastrilith are territorial and do not allow other powerful creatures to establish themselves nearby. Once they have established their territory, they do not stray far from it. They protect what is theirs and don’t meddle in matters outside their territory. Many are found on the 88th level of the Abyss, the Maw of Demogorgon, a watery plane that is dark and cold.

Wastrilith require no conventional food or drink, but they enjoy devouring creatures foolish enough to venture near their homes. They leave the remains outside to feed sharks and warn away other curiosity seekers.

Wastrilith are nasty-tempered and try to break free of the bonds imposed by spellcasters. There is a 20% chance that a summoned wastrilith ignores the wishes of the summoner, kills him, and rampages until such time as it wearies of the plane. Until that time comes, the water lord can amass considerable power beneath the waves, creating havoc in the area. It is unknown why wastrilith do this, inasmuch as they rarely take their treasure back to their home plane.

On the Prime Material, a wastrilith’s territory typically includes a 25-mile diameter circle, at the center of which is the fiend’s palace. The palace can be a marvel of undersea engineering or a simple hole in a coral reef. A wastrilith that has not established its territory will often terrorize shipping in the area, destroying boats on a whim and driving out such creatures as sea dragons and aquatic elves.

Combat: Wastrilith are canny opponents, rarely giving their enemies advantage and certainly no quarter. Because they do not believe in suffering needlessly, they send their minions forth to combat enemies: 2d6 sharks, 3d6 sahuagin, 1d4 ixitxachitl, or 1d3 water elementals. As long as the minions continue the attack and the fiend itself is not threatened, it lets them fight by themselves.

A wastrilith can breathe a blast of boiling water three times per day in a cone 30’ long with a 10’ diameter base (3d10 damage). They can attack with two claws (1d8 damage each) and bite (1d12 damage).

Electrical attacks inflict full damage. However, due to the water lords’ nature, such attacks reach them only half the time; the other half rebound on the caster, causing full damage. Fire-based attacks inflict double full damage if the fiend has left the water, but none if the creature remains fully immersed.

Water- and cold-based attacks cause no damage to the wastrilith. In addition, water elementals sent against the fiend do not attack it. Indeed, a water lord has a 50% chance of wresting control of water elementals from the summoner, as long as the water elementals have approached within 100’ of it.