Cthulhu

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

Unnatural Visage. The first time a creature sees Cthulhu they must make a DC 28 Wisdom saving throw or be stunned for 1 minute and take 2d6 psychic damage. A creature can repeat the saving throw at the end of each of its turns, or the effect ends itself after the duration ends. A creature who has been blinded or naturally blind is immune to this effect.

Abberant Speed. Whenever Cthulhu uses any movement speed, it can deal 1 psychic damage to one target it can see within 60 feet of it.

Destroyer of Minds. All Cthulhu's melee attacks deal psychic damage.

Regernation. If Cthulhu dies outside of his home demiplane, he returns to reform in 2d10 days. He is fully revived after this time, but suffers the following effects for 2d4 days:

• Cthulhu’s immunity to nonmagical bludgeoning, piercing, and slashing becomes resistance. 
• Cthulhu loses resistance to necrotic and radiant damage.
• Cthulhu’s Legendary Resistance can only be used twice a day. 
• Cthulhu’s hit point maximum is halved. 

Innate Spellacting. Cthulhu’s spellcasting ability is Charisma (DC 26, +18 to hit). Cthulhu can innately cast the following spells, requiring no material components:

at will: eldritch blast (17th level), sending, dream, dissonant whispers (2nd level)

3/day each: chromatic orb (8th level), lightning bolt, geas

1/day each: power word kill, meteor swarm, lightning bolt (9th level)

Multiattack. Cthulhu can make six attacks: two with eldritch blast, two with his fists, and two with his wings.

Fist. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit:  30 (4d10 + 8) psychic.

Wing. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit:  34 (4d12 + 8) psychic damage.

Mind Breaker. Cthulhu can choose a number of creatures equal to its Charisma modifier that it can see and are within 150 feet of it to destroy their minds. Each creature must make a DC 26 Intelligence saving throw. On a failed save the creature must take 78 (12d12) psychic damage, or half as much on a failed save. If a creature dies due to this effect, they instantly die and cannot be resurrected by any magical means, including wish, for the next 1d6 days. Then they are resurrected they suffer from 3 levels of exhaustion. Cthulhu can only use this ability again after 3d6 days.

Cthulhu can take 5 legendary actions, choosing form the options below. Only one legendary action at a time and only at the end of another creature's turn. Cthulhu regains all spent legendary actions at the start of its next turn.

Psionic Blast. Cthulhu deals 11 (2d10) psychic damage to target creature it can see.

Shockwave (Costs 2 Actions). Each creature that is within 15 feet of Cthulhu must make a DC 24 Dexterity saving throw. On a failed save a creature takes 26 (5d10) force damage and is knocked 20 feet back, falling prone. Any flying creatures must make DC 15 Dexterity saving throw or fall to the ground, taking appropriate fall damage. On a success, a creature only takes half damage and are pushed back 10 feet.

Ethereal Might. Cthulhu makes a fist attack.

Greater Mind. Cthulhu can attempt to charm a creature it can see (DC 26 Wisdom) for 1 minute.

God of the Far Realm (Costs 6 Actions). Cthulhu draws on all his might to make an all-powerful, godly attack. This takes concentration, and he can suffer the effects of concentration. Cthulhu can forgo his action to continue concentration on this attack. Cthulhu also suffers from disadvantage and a -6 penalty on the Constitution saving throw to maintain concentration. The following effects occur:

• Each creature within 10 feet of Cthulhu must make a DC 30 Constitution or Dexterity saving throw (creature's choice), taking 51 (5d20) force damage, or half as much on as successful save. Any creature that is killed by this action has disintegrated.
• Each creature within 50 feet of Cthulhu must make a DC 26 Wisdom saving throw, taking 45 (9d10) psychic damage on a failed save.
• Each creature within 200 feet of Cthulhu must make a DC 25 Strength or Dexterity (creature's choice) or take 4 (2d4) bludgeoning damage and be knocked prone.
• Each creature on the same plane of existence as Cthulhu must make a DC 23 Wisdom saving throw, taking 6 (1d10) psychic damage and gain one level of exhaustion on a failed save, or half damage on a successful one.
• After taking this legendary action, Cthulhu cannot use it again for 7 (3d4) centuries.