Cthulhu

Cthulhu can breathe air and water.

If Cthulhu dies, it revives with all its Hit Points in 1d10 weeks inside the Vault of Cthulhu.

Ground, air, and water in a 10-foot Emanation [Area of Effect] originating from Cthulhu is Difficult Terrain. A creature that starts its turn in the Emanation [Area of Effect] takes 7 (2d6) Bludgeoning damage.

If Cthulhu fails a saving throw, it can choose to succeed instead.

Cthulhu has Advantage on saving throws against spells and other magical effects.

Cthulhu makes three Eldritch Claw attacks and uses Spellcasting to cast Befuddlement if available or Mind Spike (level 4 version). It can replace one Eldritch Claw attack with a use of Tentacle.

Melee Attack Roll: +17, reach 10 ft. Hit: 18 (2d8 + 9) Slashing damage plus 9 (2d8) Acid damage. If the target is a Large or smaller creature, it has the Grappled condition (escape DC 19) from one of two limbs.

con DC 25, one creature Grappled by Cthulhu. Failure: 21 (6d6) Acid damage plus 13 (3d8) Psychic damage. Success: Half damage. Success or Failure: The target's Hit Points maximum decreases by an amount equal to the Psychic damage taken, and Cthulhu regains Hit Points equal to that amount.

Cthulhu makes an Eldritch Claw attack.

cha DC 25, each creature and object that isn't being worn or carried in a 10-foot-radius Sphere [Area of Effect] centered on a point Cthulhu can see within 60 feet. Failure: 36 (8d8) Force damage, and the target is teleported to an unoccupied space within 60 feet of Cthulhu that is horizontal to it. Success: Half damage only. Success or Failure: Cthulhu can't take this action again until the start of its next turn.

Cthulhu casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 25):

At will: Mind Spike (level 4 version)

3 each/day: Dream, Geas (can target creatures through dreams)

1 each/day: Befuddlement, Plane Shift (self only)