Arixmethes

Amphibious. Arixmethes can breathe air and water.

Hearts of the Kraken (Mythic Trait; Recharges after a Short or Long Rest). When Arixmethes is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, the damage creates cracks in its carapace, revealing its hearts. Arixmethes has four hearts: two on its chest, one on its back, and one at the base of its tail. A heart has an AC of 22 and 75 hit points. It is immune to bludgeoning, piercing, and slashing damage from nonmagical attacks, and it is immune to all conditions. If it is forced to make a saving throw, treat its ability scores as 10 (+0). If it finishes a short or long rest, the carapace heals, any destroyed hearts regenerate, and the hearts are covered again. Arixmethes dies when all the hearts are destroyed. 

Spell-Resistant Carapace. Arixmethes has advantage on saving throws against spells, and any creature that makes a spell attack against Arixmethes has disadvantage on the attack roll.

Legendary Resistance (3/Day). If the Arixmethes fails a saving throw, he can choose to succeed instead.

Magic Weapons. Arixmethes’s weapon attacks are magical.

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

Multiattack. Arixmethes makes three attacks: one with its tail, and two with its tentacle grasp.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 21 (2d10 + 10) bludgeoning damage, and if the target is a creature, it must succeed on a DC 25 Strength saving throw or be knocked prone.

Tentacle Grasp. Melee Weapon Attack: +17 to hit, reach 20 ft., one creature. Hit: 19 (2d8 + 10) bludgeoning damage, and the target is grappled (escape DC 25). If the target doesn’t escape by the end of its next turn, Arixmethes throws the target up to 30 feet in a straight line. The target lands prone and takes 10 (3d6) bludgeoning damage.

Bite. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 29 (3d12 + 10) piercing damage. If the target is a Large or smaller creature grappled by Arixmethes, 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 Arixmethes, and it takes 35 (10d6) acid damage at the start of each of Arixmethes turns. If Arixmethes takes 50 damage or more on a single turn from a creature inside it, Arixmethes must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Arixmethes. If Arixmethes dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Arixmethes can take 3 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. Arixmethes regains spent legendary actions at the start of its turn.

Attack. Arixmethes makes one Tentacle Grasp or Tail attack.

Move. Arixmethes moves up to its speed. If Arixmethes is swimming, this movement doesn't provoke opportunity attacks.

Shocking Aura (Costs 3 Actions). Arixmethes discharges electrical currents into the surrounding water. Until the start of Arixmethes's next turn, whenever a creature starts its turn underwater within 20 feet of Arixmethes, that creature must succeed on a DC 24 Constitution saving throw or take 45 (10d8) lightning damage.

If Arixmethes's mythic trait is active, it can use the options below as legendary actions for 1 hour after using Hearts of the Kraken.

Rampage. Arixmethes makes two attacks: one with its tail and one with its tentacle grasp.

Steam Vents (Costs 2 Actions). Each creature within 10 feet of Arixmethes must make a DC 24 Constitution saving throw, taking 13 (3d8) fire damage on a failed save, or half as much damage on a successful one. Until the start of its next turn, Arixmethes's hearts are heavily obscured giving attacks against them disadvantage.