Bowser

Giant Bowser (Recharges after a Short or Long Rest). If Bowser would be reduced to 0 hit points, his current hit point total instead resets to 254 hit points, and he recharges his Bowser Breath. In addition, Bowser gains the effects of Enlarge/Reduce (enlarge only) whilst this trait is active. Additionally, Bowser can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 8,400 XP (16,800 XP total) for defeating Bowser after his Giant Bowser activates.

Hold Breath. Bowser can hold his breath for 1 hour.

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

Powerful Build. Bowser counts as one size larger when determining his carrying capacity and the weight he can push, drag, or lift.

Multiattack. Bowser makes three Claw attacks, Slam attacks, Fireball attacks, or a combination of the three.

Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 18 (3d6 + 8) slashing damage. Objects and structures take double damage.

Fireball. Ranged Weapon Attack: +13 to hit, range 100/300 ft., one target. Hit: 12 (5d4) fire damage. If the target is a flammable object, it ignites. Until a creature takes an action to douse the fire, the target takes 5 (1d10) fire damage at the start of each of its turns.

Slam. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 19 (2d10 + 8) slashing damage and the target must make a DC 20 Strength saving throw or either be pushed 20 feet away from Bowser or be knocked prone (Bowser's choice).

Bowser Breath (Recharge 5-6). Bowser uses one of the following breath weapons.

Concentrated Fire Breath. Bowser exhales fire in a 90-foot line that is 5 feet wide. Each creature in that line must make a DC 21 Dexterity saving throw, taking 54 (12d8) fire damage on a failed save, or half as much damage on a successful one.

Fire Rain Breath. Bowser exhales a flaming ball of fire at a point he can see within 120 feet which then explodes in a rain of fire which descends in a 20-foot-radius, 40-foot-high cylinder. Each creature in the cylinder must make a DC 21 Dexterity saving throw, taking 28 (8d6) fire damage on a failed save, or half as much damage on a successful one. In addition, the ground within the cylinder ignites until the start of Bowser's next turn. Until the flames subside, the area becomes difficult terrain and when a creature moves into or within the area, it takes 5 (2d4) fire damage for every 5 feet it travels. Creatures with immunity to fire damage are immune to this effect.

Homing Fireball Breath. Bowser exhales a blue fireball at one creature he can see within 300 feet that isn't behind total cover. The creature must make a DC 21 Dexterity saving throw or take 27 (5d10) fire damage on a failed save. If the creature is within 150 feet of Bowser and succeeds on its initial saving throw, it must repeat the saving throw with advantage, taking 17 (5d6) fire damage on a failed save.

King's Shell. Bowser withdraws or emerges from his shell. Until he emerges, he gains a +4 bonus to his AC, his damage resistances become immunities, and he has advanatge on Strength and Constitution saving throws. Whilst in his shell, Bowser's movement speed is 0, he has disadvantage on Dexterity saving throws, can't take reactions, actions, or bonus actions other than King's Shell, and cannot use any Legendary Actions besides Whirling Fortress or Shell Swap. In addition, each creature within 5 feet of Bowser that hits him with a melee attack whilst he is in his shell must make a DC 21 Dexterity saving throw or take 9 (2d8) piercing damage.

Roar. Bowser emits a roar that can be heard from up to 1 mile away. Each creature within 120 feet of Bowser that isn't an ally and can hear the roar must make a DC 15 Wisdom saving throw or become frightened of Bowser until the start of Bowser's next turn. A creature that succeeds on its saving throw has advantage on subsequent saving throws against Bowser's Roar for the next hour.

Heavy Impact. When Bowser takes damage from falling, he can reduce the damage taken by 60 points. Each creature on the ground within 15 feet of Bowser must make a DC 20 Dexterity saving throw or be knocked prone.

Tough Guy. When Bowser takes damage, he can expend a hit die to reduce the damage by 14 (1d10 + 9). Bowser can expend up to four additional hit dice, each hit dice adding an additional 1d10 to the roll.

Bowser 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. Bowser regains spent legendary actions at the start of his turn.

Advance. Bowser moves up to half his movement speed.

Swipe. Bowser makes one Claw attack.

Bowser Bomb (Costs 2 Actions). Bowser leaps into the air without provoking opportunity attacks before landing at a point he can see within 30 feet of him without taking damage. Each creature in a 5-foot-radius, 30-foot-high cylinder centred on this point must succeed on a DC 20 Dexterity saving throw. On a successful save, the creature takes no damage and relocates to an unoccupied space of its choice within 5 feet of Bowser. On a failed save, the creature takes 27 (6d8) bludgeoning damage, falls prone and until Bowser uses Bowser Bomb again or moves, the creature is restrained. While restrained this way, the creature (or another creature within 5 feet of it) can use its action to make a DC 20 Strength check. On a success, the creature relocates to an unoccupied space of its choice within 5 feet of Bowser 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).

Fire (Costs 2 Actions). Bowser makes one Fireball attack.

Whirling Fortress (Costs 3 Actions). Whilst Bowser is withdrawn in his shell, he moves up to 60 feet in any direction without provoking opportunity attacks. When Bowser moves within 5 feet of a creature or an object that isn't being worn or carried, it must make a DC 21 Dexterity saving throw or take 27 (5d10) slashing damage on a failed save. A creature or object can take this damage only once during a turn.

If Bowser's Giant Bowser trait has activated in the last hour, he can use the options below as legendary actions.

Furious Swipes. Bowser makes two Claw attacks.

Shell Swap. Bowser uses King's Shell.

Koopa King's Wrath (Costs 3 Actions). Bowser charges in a straight line that is up to 60 feet long. Each creature in Bowser's path must make a DC 20 Dexterity saving throw or take 27 (5d10) bludgeoning damage on a failed save. Any objects or structures in Bowser's path take the maximum damage. Once Bowser has moved to the end of the line, one creature of Bowser's choice that he can see within 5 feet of him must make a DC 20 Dexterity saving throw or take 34 (4d12 + 8) bludgeoning damage, be knocked prone, and be pushed 30 feet away from Bowser.