Legendary Resistance (3/Day). If Zetsukaijin fails a saving throw, it can choose to succeed instead.

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

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

Amphibious. Zetsukaijin can breathe air and water.

Magic Weapons. Zetsukaijin’s weapon attacks are magical.

Regeneration. If Zetsukaijin has at least 1 hit point, it regains 20 hit points at the start of its turn. If Zetsukaijin takes cold damage, Zetsukaijin must make a DC 25 Consitution saving throw or have its regeneration disabled until the end of its next turn.

Multiattack. Zetsukaijin can use its Frightful Presence. It then makes four attacks: one with its bite, two with its claws and one with its tail.

Bite. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 27). Until this grapple ends, the target is restrained, and Zetsukaijin can't bite another target.

Claw. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 24 (4d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 30 ft., one target. Hit: 28 (4d8 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of Zetsukaijin's choice within 120 feet of it and aware of it must succeed on a DC 22 Wisdom saving throw or become frightened for 1 hour. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if Zetsukaijin 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 Zetsukaijin's Frightful Presence for the next 24 hours.

Atomic Weapon (Recharge 5-6). Zetsukaijin uses one of the following Atomic weapons.

Atomic Breath. Zetsukaijin exhales an atomic beam in a 240-foot line that is 10 feet wide. Each creature in that line must make a DC 27 Dexterity saving throw, taking 121 (22d10) force damage on a failed save, or half as much damage on a successful one. In addition, if the creature fails that save by 5 or more, Zetsukaijin can choose to either knock the creature prone or push the creature 30 feet away.

Atomic Pulse. Zetsukaijin releases a pulse of atomic energy in a 30-foot radius around itself. Every creature in the area must make a DC 27 Consutiton saving throw. On a failure, the creature takes 44 (8d10) force damage, is pushed 30 feet away from Zetsukaijin and knocked prone. On a success, the creature takes half damage and is pushes 10 feet away.

Zetsukaijin can take 4 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. Zetsukaijin regains spent legendary actions at the start of its turn.

Attack. Zetsukaijin makes one claw attack.

Move. Zetsukaijin moves up to half its movement speed.

Stomp (Costs 2 Actions). Zetsukaijin stomps one of its feet at a point on the ground within 15 feet of it. Any creature in a 20-foot-radius, 20-foot-high cylinder centred on this point must succeed on a DC 27 Dexterity saving throw or take 33 (4d10) bludgeoning damage and fall prone. Until Zetsukaijin uses its Stomp again or moves, the creature is restrained. While restrained in this way, the creature (or another creature within 5 feet of it) can use its action to make a DC 27 Strength check. On a success, the creature relocates to an unoccupied space of its choice within 5 feet of Zetsukaijin and is no longer restrained.

Structures, as well as nonmagical objects that are neither being worn nor carried, take the same amount of damage if they are in the cylinder (no save).

Bellowing Roar (Costs 2 Actions). Zetsukaijin lets out a thunderous roar. All creatures within 30 feet of Zetsukaijin must make a DC 27 Consitution saving throw. On a failure, the creature takes 35 (10d6) thunder damage and is deafened for 1 minute. If a creature fails the save by 10 or more, it is also stunned for 1 minute. On a success, the creature takes half damage. A creature can reroll the saving throw at the end of its next turn, ending the effect on a success.

Burning Alpha (Mythic Trait; Recharges after a Short or Long Rest). When Zetsukaijin is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, it starts to glow red and emit heat, regaining 295 hit points. In addition, any creature within 30 feet of Zetsukaijin that starts its turn or on its turn enters the area must make a DC 27 Consitution saving throw, taking 28 (8d6) fire damage on a failure or half as much on a success. Also, Zetsukaijin gives off bright light in a 30-foot radius, and dim light for an additional 30 feet, his atomic abilities taking on a red colour instead of their usual blue.

If Zetsukaijin's mythic trait is active, it can use the options below as legendary actions for 1 hour after using Burning Alpha:

Swipe. Zetsukaijin makes two claw attacks.

Dash. Zetsukaijin moves up to its full movement speed.

Fling (Costs 2 Actions). Zetsukaijin makes one tail attack. On a hit, the creature takes the tail's damage and must make a DC 27 Strength saving throw. On a failure, the creature is knocked 60 feet away. Once the creature lands, regardless of whether it went the full 60 feet or not, it takes 21 (6d6) bludgeoning damage from the impact. 
If the creature has travelled further than 60 feet after being pushed, it takes an additional 3 (1d6) bludgeoning damage per 10 additional feet travelled to a maximum of 49 (14d6) additional bludgeoning damage. The creature lands prone unless it has a way to negate taking damage from the fall.

Meltdown (Costs 3 Actions). Zetsukaijin recharges and uses its Atomic Weapon.