Multi-Attack. Cthulhu can make two claw attacks, and one bite attack.

Bite. Melee Weapon Attack: +25 to hit, reach 15 ft., 1 target. Hit: (14d12 + 19) piercing and bludgeoning damage.  If the target is a large or smaller creature that is grappled by Cthulhu, that creature is also swallowed and the grapple ends. While swallowed, the creature is blinded and restrained. It has total cover from outside attacks and effects. It take (12d12) acid damage at the beginning of each of Cthulhu's turns. If cthulhu take 80 damage or more on a single turn from a creature inside it. cthulhu must succeed on a constitution saving throw dc 25 at the end of its turn, or regurgitate all swallowed creatures within a 15 ft space around it.

Claw. Melee Weapon Attack: +25 to hit, reach 30 ft., 1 target. Hit: (10d10 + 19) slashing and bludgeoning damage and the target is grappled (DC 33). Until the grapple ends the target is restrained. 

Fling. One large or smaller object held or grappled by cthulhu is thrown up to 300 ft in a direction of its choosing and is knocked prone. If a thrown target strikes a solid object it take (1d6)  bludgeoning damage for every ten feet it was thrown. If the target is thrown at another creature, the creature must make a DC 33 dex saving throw, or take the same damage and be knocked prone.

Enter the description for how legendary actions work for your monster here.

Move. Cthulhu can move up to half its speed.

Regenerate. Cthulhu regenerates 50 HP.

Attack. Cthulhu makes two claw attacks and one bite attack