Legendary Resistance (12/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.
Malignancy. The amalgamation can’t be teleported or travel to another plane against its will.
Immutable Form. The amalgamation is immune to any spell or effect that would alter its form.
Skin of the Aberrant. Any time Cthulhu is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 5-6, Cthulhu is unaffected.
Siege Monster. Cthulhu deals double damage to objects and structures.
Multiattack. Cthulhu can use its Frightful Presence. It then makes four attacks: one tentacle, two with its claws, and one bite.
Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the Cthulhu can’t bite another target.
Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.
Tentacle. Melee Weapon Attack: +19 to hit, reach 50 ft., one target. Hit: 20 (3d10 + 10) bludgeoning damage, and the target is grappled (escape DC 27). Until this grapple ends, the target is restrained. Cthulhu has twelve tentacles, each of which can grapple one target.
Mind-breaking Whispers. Cthulhu utters a terrible truth within the mind of one creature within 120 feet of it. That creature must succeed on a DC 27 Intelligence saving throw. On a failure, it takes 65 (10d6+30) psychic damage and becomes stunned until the end of it’s next turn.
Frightful Presence. Each creature of the Cthulhu’s choice within 120 feet of it and aware of it must succeed on a DC 21 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Cthulhu is within line of sight, 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 the Cthulhu’s Frightful Presence for the next 24 hours.
Contaminating Presence. Creatures within 60 feet of Cthulhu must succeed on a DC 20 Constitution saving throw or take 21 (6d6) necrotic damage and gain one level of contamination.
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. Cthulhu regains spent legendary actions at the start of its turn.
Tentacle Attack. Cthulhu makes one tentacle attack.
Mind-breaking Whispers (Costs 2 Actions). Cthulhu uses Mind-breaking Whispers.