Amphibious. Cthulhu can breathe air and water

Call of Cthulhu (Mythic Trait; Recharges after a Short or Long Rest). If Cthulhu would be reduced to 0 hit points, its current hit point total instead resets to 500 hit points. Additionally, Cthulhu can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating Cthulhu after its Call of Cthulhu activates.

Immortality. If Cthulhu is killed, its body immediately fades away into a noxious cloud of otherworldly vapor that fills an area out to its reach. This cloud blocks vision as fog cloud, but can’t be dispersed by any amount of wind. Any creature in this area must succeed at a DC 27 Constitution saving throw or be poisoned for as long as it remains in the cloud and for an additional 1d10 rounds after it leaves the area. Cthulhu returns to life after 2d6 rounds, manifesting from the cloud and restored to life via resurrection, but has 5 levels of exhaustion and cannot take both a move and action in the same turn. At the end of each of its turns, Cthulhu makes a DC 20 Constitution saving throw to reduce its exhaustion level by 1. If slain again while suffering from this effect, Cthulhu reverts to vapor form again and its essence fades away after 2d6 rounds, returning to its birth place, Vhoorl.

Innate Spellcasting. Cthulhu's spellcasting ability is Wisdom (spell save DC 27, +19 to hit with spell attacks). He can innately cast the following spells, requiring no material components:

At will: astral projection, control weather, dispel magic, dream, phantasmal killer, sending, suggestion, teleport

3/day each: antipathy/sympathy, feeblemind, gate, plane shift, weird

1/day: power word stun, symbol, wish

Insanity. Any creature that attempts to interact directly with Cthulhu’s thoughts (such as via detect thoughts or telepathy) must succeed at DC 27 Wisdom saving throw or gain an indefinite madness. When using its telepathy to communicate Cthulhu doesn’t activate this feature unless he spends an action to focus its mind on one opponent.

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

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

Nightmare Haunting. While on the Ethereal Plane or Material Plane, Cthulhu magically touches a sleeping humanoid on the Material Plane. A protection from evil and good spell cast on the target prevents this contact, as does a magic circle. As long as the contact persists, the target has dreadful visions. If these visions last for at least 1minute, the target gains no benefit from its rest, and its hit point maximum is reduced by 7 (2d6). If this effect reduces the target’s hit point maximum to 0, the target dies and its soul travels to Vhoorl. The reduction to the target’s hit point maximum lasts until removed by the greater restoration spell or similar magic. For every 5 minutes the visions last, the target gains a short-term madness. For every 10 minutes the visions last, the target gains a long-term madness.

Regeneration. Cthulhu regains 30 hit points at the start of its turn if he has at least 1 hit point.

Starflight. Cthulhu can survive in the void of outer space and flies through the cosmos at incredible speeds. The exact travel time varies from one trip to the next, but a trip within a solar system normally takes Cthulhu 2d6 hours, and a trip beyond normally takes 2d6 days (or more, at the GM’s discretion).

Multiattack. Cthulhu makes three tentacle attacks, each of which he can replace with one use of Fling.

Bite. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 23 (3d8 + 10) piercing damage. If the target is a Large or smaller creature grappled by Cthulhu, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Cthulhu, and it takes 42 (12d6) acid damage at the start of each of Cthulhu's turns.

If Cthulhu takes 50 damage or more on a single turn from a creature inside him, Cthulhu must succeed on a DC 25 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 him and can escape from the corpse using 15 feet of movement, exiting prone.

Tentacle. Melee Weapon Attack: +19 to hit, reach 30 ft., one target. Hit: 22 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 20). Until this grapple ends, the target is restrained. Cthulhu has ten tentacles, each of which can grapple one target.

Fling. One Large or smaller object held or creature grappled by Cthulhu is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 20 Dexterity saving throw or take the same damage and be knocked prone.

Lightning Storm. Cthulhu magically creates three bolts of lightning, each of which can strike a target Cthulhu can see within 120 feet of him. A target must make a DC 25 Dexterity saving throw, taking 22 (4d10) lightning damage on a failed save, or half as much damage on a successful one.

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 or Fling. Cthulhu makes one tentacle attack or uses its Fling.

Lightning Storm (Costs 2 Actions). Cthulhu uses Lightning Storm.

Ink Cloud (Costs 3 Actions). While underwater, Cthulhu expels an ink cloud in a 60-foot radius. The cloud spreads around corners, and that area is heavily obscured to creatures other than Cthulhu . Each creature other than Cthulhu that ends its turn there must succeed on a DC 23 Constitution saving throw, taking 16 (3d10) poison damage on a failed save, or half as much damage on a successful one. A strong current disperses the cloud, which otherwise disappears at the end of Cthulhu's next turn.

If Cthulhu's mythic trait is active, he can use the options below as legendary actions for 1 hour after using Call of Cthulhu.

Mind-Shattering Presence (Costs 3 Actions). Each creature of Cthulhu’s choice within 300 feet of him and aware of him must succeed on a DC 17 Wisdom saving throw or die from terror. A creature immune to fear that fails its saving throw is stunned 1d4 rounds instead of killed. On a success, a creature is immune to Cthulhu’s Mind-Shattering presence for 1 minute.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 25 (3d8 + 10) slashing damage, and the target is grappled (escape DC 20). Until this grapple ends, the target is restrained. Cthulhu has two claws, each of which can grapple one target.

Armor of Storms (Costs 2 Actions). Lightning temporarily surrounds Cthulhu, and he gains 40 temporary hit points until the start of its next turn. Until all these temporary hit points are gone, any creature that touches Cthulhu or hits him with a melee attack takes 26 (4d12) lightning damage.