Cthulhu

Amphibious. The cthulhu can breathe air and water.

Natural Slime. While underwater, the cthulhu is surrounded by cloud of slime. A creature that touches the chuthulhu or that hits it with a melee attack while within 5 feet of it must make a DC 14 Constitution saving throw. On a failure, the creature is diseased for 2d6 hours. The diseased creature can breathe only underwater.

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.

Immortal. Cthulhu has achieved immortality, so it cannot age  

Multiattack. The cthulhu makes three tentacle attacks.

Tentacle. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage plus 12(2d6+5) psychic damage. If the target is a creature, it must succeed on a DC 20 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 bloated 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.

Mind Control. Ranged Weapon Attack:+10 to hit, reach 25 ft.,one target. Hit: 6(1d12) psychic damage. If the target is a creature with an intelligence score of 10 or higher, it must succeed on a DC20 Intelligence saving throw or become under controll of the cthulhu

Tail. Melee Weapon Attack: +9 to hit, reach 10 ft. one target. Hit: 15 (3d6 + 5) bludgeoning damage.

Enslave (5/Day). The cthulhu targets one creature it can see within 30 feet of it. The target must succeed on 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.

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.

The cthulhu can take 5 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.

Detect. The cthulhu makes a Wisdom (Perception) check.

Tail Swipe. The cthulhu makes one tail attack.

Psychic Drain (Costs 2 Actions). One creature charmed by the cthulhu takes 12(2d6+5) psychic damage, and the cthulhu regains hit points equal to the damage the creature takes.