Cthulhu

Traits

Amphibious. Cthulhu can breathe air and water.

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

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

Legendary Resistance (4/Day, or 5/Day in Domain). If Cthulhu fails a saving throw, it can choose to succeed instead.

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

Actions

Multiattack. 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.

Eldritch Claw. 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 19) from one of two limbs.

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

Bonus Actions

Warp. Cthulhu teleports to an unoccupied space it can see within 120 feet, taking creatures it is grappling with it. Then each creature within 10 feet of the space it left is subjected to the following effect. str 25. Failure: 7 (2d6) Bludgeoning damage plus 7 (2d6) Thunder damage, and if the target is a Large or smaller creature, it has the Prone condition. Success: Half damage only.

Legendary Actions

Clamp. Cthulhu makes an Eldritch Claw attack.

Fold Space. cha 25, each creature and object that isn't being worn or carried in a 10-foot-radius Sphere 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. Failure or Success: Cthulhu can't take this action again until the start of its next turn.