Cthulhu

Amphibious. The Cthulhu can breathe air and water.

Probing Telepathy. If a creature communicates telepathically with the Cthulhu, the Cthulhu learns the creature’s greatest desires if the Cthulhu can see the creature.

Origin of Madness. Once a creature sees Cthulhu for the first time, they need to make a reverse Int ST of 17 or become inflicted with madness. 

Legendary Resistance. If Cthulhu fails a saving throw, it can choose to succeed instead twice per day. 

Eyes See All. Cthulhu knows the location and is aware of every life form within 1 mile.

Multiattack. The Cthulhu makes five tentacle attacks.

Tentacle. Melee Weapon Attack: +12 to hit, reach 20ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage. If the target is a creature, it must succeed on a DC 14 Constitution saving throw or become diseased. The disease has no effect for 1 minute and can be removed by any magic that cures disease. After 1 minute, the diseased creature’s skin becomes translucent and slimy, the creature can’t regain hit points unless it is underwater, and the disease can be removed only by heal or another disease-curing spell of 6th level or higher. When the creature is outside a body of water, it takes 6 (1d12) acid damage every 10 minutes unless moisture is applied to the skin before 10 minutes have passed.

Enslave (3/Day). The Cthulhu targets one creature it can see within 30 feet of it. The target must succeed on a DC 20 Wisdom saving throw or be magically charmed by the Cthulhu until the Cthulhu dies or until it is on a different plane of existence from the target. The charmed target is under the Cthulhu's control and can’t take reactions, and the Cthulhu and the target can communicate telepathically with each other over any distance.

Whenever the charmed target takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from the Cthulhu.

Psionic Wave (Recharge 5 turns) - Cthulhu builds up energy in his eyes and blasts psychic energy in a 120' cone originating at his eye. Each target must make an Int ST or suffer 60 (10d6 + 30) psychic damage. 

The Cthulhu 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 Cthulhu regains spent legendary actions at the start of its turn.

Tail Swipe. The Cthulhu makes one tentacle attack.

Psychic Drain (Costs 2 Actions). One creature charmed by the Cthulhu takes 12 (4d6) psychic damage and the Cthulhu regains hit points equal to the damage the creature takes.

Rise of Cthulhu [Recharge 3 turns](Costs 3 Actions). Cthulhu reaches his hands into the sky and emits a wave of psychic energy from the moon. Every creature within one mile of Cthulhu must make a DC 14 Wisdom saving throw or be magically charmed by the Cthulhu until the Cthulhu dies or until it is on a different plane of existence from the target. The charmed target is under the Cthulhu's control and can’t take reactions, and the Cthulhu and the target can communicate telepathically with each other over any distance.