Cthulhu

Amphibious. Cthulhu can breath air and water.

Sunlight Sickness. If Cthulhu is in complete and direct sunlight, and is in its true form, it has disadvantage on perception checks and saving throws.

Ugliness: Cthulhu automatically fails any Charisma check except Intimidation checks and DC 10 or lower saving throws.

Shapeshifter. Cthulhu uses one action to polymorph into a large squid, medium humanoid or back into its true form. When in squid form, Cthulhu can't use stomp and when in humanoid, can't use tentacles or reel. If Cthulhu is in squid or true form and turns into its humanoid form, all creatures grappled by it are ungrappled. It reverts to it's true form if it dies.

Spellcasting. The Cthulhu is a 13th level spellcaster. Its spellcasting ability is Wisdom (spell save DC 15, +7 to hit with spell attacks). The Cthulhu has following spells prepared:
6th level (3 slots):  Flesh to stone, circle of death, eye bite
7th level (3 slots):  Finger of death

Stomp (2/day). Melee Weapon Attack: +12 to hit, targets (all creatures within 200ft). Hit: 37 (4d20 + 15) bludgeoning damage and 11 (2d6+4) piercing damage.

Tentacle. RangedWeapon Attack: +5 to hit, range 20/150 ft., 3 targets. Hit: 29 (3d12 + 10) bludgeoning damage and 19 (3d6) acid damage and the target is grappled (escape DC 19). Cthulhu can grapple 6 creatures at a time.

Reel. All creatures grappled by Cthulhu must make a DC 17 Strength (Athletics) check, moving 40ft towards Cthulhu on a failure, or 20ft on a success.

Squash. If Cthulhu takes 45 or more damage in one go from a creature 20ft or less away, it makes a stomp attack with no take away from its twice a day feature.

Attack. Cthulhu attacks with tentacles or reel.

Dodge. Cthulhu hides, dodges or disengages.

Submerge. Next time Cthulhu submerges, it teleports to its lair.

Recover. Cthulhu gains 10 HP and all used spell slots.