Avatar of Cthulhu

Legendary Resistance (3/Day). If the avatar fails a saving throw, it can choose to succeed instead.

Magic Resistance. The avatar has advantage on saving throws against spells and other magical effects.

Psychic Mirror. If the avatar takes psychic damage, each creature within 10 feet of the hulk takes that damage instead; the avatar takes none of the damage. In addition, the avatar’s thoughts and location can’t be discerned by magic.

Innate Spellcasting (Psionics). The avatar is a 20th-level spellcaster. Its innate spellcasting ability is Wisdom (spell save DC 21; +13 to hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: mind sliver, infestation, vicious mockery, eldritch blast, true strike

1st level (4 slots): dissonant whispers, chaos bolt, bane, charm person

2nd level (3 slots): tasha's mind whip, phantasmal force, mind spike

3rd level (3 slots): feign death, counterspell, hold person

4th level (3 slots): phantasmal killer, hallucinatory terrain, banishment

5th level (3 slots): synaptic static, geas,dominate person

6th level (2 slots): mental prison, disintegrate

7th level (2 slots): mirage arcane, teleport

8th level (1 slots): feeblemind

9th level (1 slots): psychic scream

Multiattack. The avatar makes three tentacle attacks. If two or more attacks hit the same target, the target also takes 18 (4d8) psychic damage and must succeed on a DC 20 Constitution saving throw or be stunned until the end of the target’s next turn.

Tentacles. Melee Weapon Attack: +11 to hit, reach 10 ft., one creature. Hit: 26 (4d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 20) and must succeed on a DC 18 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon Attack: +11 to hit, reach 10 ft., one incapacitated humanoid grappled by the avatar. Hit: The target takes 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the avatar kills the target by extracting and devouring its brain.

The avatar can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The avatar regains spent legendary actions at the start of its turn.

Cantrip. The avatar casts a cantrip.

Cast a Spell. The avatar casts a spell.

Energy Drain (Costs 2 Actions). Each creature within 30 feet of the avatar must make a DC 15 Constitution saving throw. On a failed save, the creature’s hit point maximum is magically reduced by 10 (3d6). If a creature’s hit point maximum is reduced to 0 by this effect, the creature dies. A creature’s hit point maximum can be restored with the greater restoration spell or similar magic.

Detention (Costs 3 Actions). The avatar targets a creature it can see within 60 feet of it. The target must succeed on a DC 18 Charisma saving throw or be magically teleported to a harmless demiplane until the end of the avatar’s next turn, whereupon the target reappears in the space it left or the nearest unoccupied space if that space is occupied.