Cthulhu

Discorporation. When Cthulhu drops below 0 hit points, it is not destroyed it is simply banished back to its prison in R’lyeh and unable to take any actions until released.

Innate Spellcasting (Psionics). Cthulhu’s innate spellcasting ability is Intelligence (spell save DC 26, +18 to hit with spell attacks). Cthulhu can innately cast the following spells, requiring no material components:

At will: Charm Person, Control Water, Detect Evil and Good, Detect Magic, Detect Thoughts, Dispel Magic, Dissonant Whispers, Feeblemind, Modify Memory, Phantasmal Force, Sending, Telekinesis, Telepathy

3/day each: Control Weather, Dominate Monster, Dream, Plane Shift, Teleport, Weird

1/day each: Astral Projection, Etherealness, Time Stop

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

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

Magic Weapons. Cthulhu's attacks are considered magical.

Non–Euclidean Madness. A creature that starts its turn within 210 feet of Cthulhu and is aware of it must succeed on a DC 25 Wisdom saving throw. On a failed save, a creature must roll on the Short–Term Madness table in the Dungeon Master’s Guide for a random effect. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to Cthulhu’s Non–Euclidean Madness for the next 24 hours.

Other Worldly Nature. Cthulhu doesn’t require air, food, or water.

Regeneration. Cthulhu regains 25 hit points at the start of its turn if he has at least 1 hit point.

Siege Monster. Cthulhu deals double damage to objects and structures.

Multiattack. Cthulhu uses its Otherworldly Gaze. It then makes four attacks: two slam attacks one tail attack and one tentacle attack. Cthulhu can use Swallow instead of its tentacle attack. Alternatively, Cthulhu makes one attack: one with its tail and casts one spell.

Slam. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 28 (4d8 + 10) bludgeoning damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 10) bludgeoning damage. On a hit the target creature must succeed on a DC 27 Strength saving throw or fall prone.

Tentacle. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) bludgeoning damage plus 11 (2d10) psychic damage. If the target is a large or smaller creature, it is grappled and restrained (escape DC 20). Until this grapple ends, Cthulhu can't use its tentacle on another target.

Otherworldly Gaze. Cthulhu turns its gaze towards a creature within 150 feet that Cthulhu can see, forcing the target to make a DC 25 Intelligence saving throw. On a failed save, the target takes 22 (4d10) psychic damage and is stunned until the start of Cthulhu’s next turn.

Swallow. Cthulhu makes a tentacle attack against a Large or smaller target it is grappling. On a hit, the creature takes the normal damage and is swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside of Cthulhu, and it takes 35 (10d6) acid damage at the start of each of Cthulhu’s turns.

If Cthulhu takes 40 damage or more on a single turn from creatures inside it, Cthulhu must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Cthulhu. If Cthulhu dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 20 feet of movement, exiting prone.

Eldritch Howl (Recharge 5–6). Cthulhu emits a soul–splintering howl. All creatures within 150 feet of Cthulhu and can hear it must make a DC 27 Constitution saving throw. On a failed save, a target takes 63 (6d20) thunder damage and is deafened until the start of Cthulhu's next turn. Half as much damage and not deafened on a successful saving throw.

Also, each creature of Cthulhu’s choice within 150 feet of it must make a DC 27 Wisdom saving throw. On a failed save, a creature takes 63 (6d20) psychic damage and is frightened until the start of Cthulhu’s next turn. Half as much damage and not frightened on a successful saving throw.

Being deafened does not protect against psychic damage and grants immunity to thunder damage.

Enslavement (Recharges 5–6). Cthulhu telepathically speaks unknown arcane words of domination into the mind of a creature it is aware of within 150 feet. The target must make a DC 26 Wisdom saving throw. On a failed save, the creature is magically charmed by Cthulhu until Cthulhu's physical form is destroyed. The charmed creature follows all of Cthulhu’s commands to the best of their capabilities.

If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Cthulhu's Enslavement for the next 24 hours.

Cthulhu can take 5 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 its turn.

Attack. Cthulhu makes one slam attack.

Cast Spell (Cost 2 Actions). Cthulhu casts a spell using a slot as normal.

Unspeakable Horror (Costs 3 Actions). Cthulhu consumes the soul of an unconscious creature within 60 feet of it that Cthulhu can see. Cthulhu gains 31 (2d20 + 10) temporary hit points, and the creature dies and cannot be resurrected except via a wish spell or until Cthulhu chooses to release its soul or until Cthulhu’s physical form is destroyed.