Avatar of Cthulhu, the Dreaming Depth

Multiattack. Cthulhu makes two Eldritch Claw attacks and one Tentacled Maw attack.

Eldritch Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 9) slashing damage plus 36 (8d8) psychic damage. 

Tentacled Maw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 9) slashing damage plus 36 (8d8) psychic damage. If the target is large or smaller it is grappled (escape DC 20) and must succeed on a DC 20 Intelligence saving throw or become stunned until this grapple ends.

Otherworldly Howl (Recharge 5-6). Cthulhu emits a soul-splintering howl. Each creature in a 60-foot radius around Cthulhu must make a DC 23 Constitution saving throw, taking 33 (6d10) thunder damage and 33 (6d10) psychic damage on a failed save and being deafened until the end of its next turn, or half as much damage on a successful one. Not being able to hear does not protect against this attack. Creatures have disadvantage on this saving throw if they are grappled by Cthulhu's Tentacled Maw. 

Innate Spellcasting. Cthulhu's spellcasting ability is Charisma (spell save DC 24). He can innately cast the following spells, requiring no material components:

Cthulhu can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Cthulhu regains spent legendary actions at the start of his turn.

Teleport. Cthulhu uses Teleport.

Claw (Costs 2 Actions). Cthulhu makes one Eldritch Claw attack.

Maddening Gaze (Costs 2 Actions). Cthulhu turns his magical gaze toward one creature he can see within 120 feet of him and forces maddening secrets into its mind. The target must succeed on a DC 21 Wisdom saving throw or take 22 (4d10) psychic damage.

Scion of Insanity (Costs 3 Actions). Each creature within 60 feet of Cthulhu must succeed on a DC 21 Wisdom saving throw or use its reaction to make a melee weapon attack against a random creature within reach. If no creatures are within reach, it makes a ranged weapon attack against a random creature within range, throwing its weapon if necessary. This attack is made with advantage and gains a +4 bonus to the damage roll.