Cataclysmic Tarrasque

 Legendary Resistance (5/Day). If the tarrasque fails a saving throw, it can choose to succeed instead.

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

Magic Weapons. The tarrasque's weapon attacks count as magical for the purpose of overcoming resistance and immunity.

Reflective Carapace. Any time the tarrasque is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 4, the tarrasque is unaffected. On a 5-6, the tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the tarrasque, turning the caster into the target.

Siege Monster. The tarrasque deals double damage to objects and structures.

World Ender (Recharges after a Short or Long Rest).  If the tarrasque would be reduced to 0 hit points, its current hit point total instead resets to 550 hit points and it regains any expended uses of Legendary Resistance. Additionally, the tarrasque 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 the tarrasque after its World Ender activates.

Multiattack. The tarrasque makes four attacks: one with its bite, two with its claws, and one with its tail. It can use its Swallow instead of its 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 tarrasque can’t bite another target.

Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8+10) slashing damage.

Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 32 (4d10+10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Swallow. The tarrasque makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the tarrasque, and it takes 56 (16d6) acid damage at the start of each of the tarrasque’s turns.

If the tarrasque takes 70 damage or more on a single turn from a creature inside it, the tarrasque must succeed on a DC 27 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the tarrasque. If the tarrasque dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Cataclysm (World Ender Only; Recharges after a Short or Long Rest). The tarrasue roars and calls forth a great cataclysm, shattering the earth around it and bringing forth titanic, ruinous storms. Until the tarrasque is killed, each object and creature other than the tarrasque within one mile of the it takes 7 (2d6) thunder, fire, lightning, bludgeoning, piercing, and slashing damage at the start of each of their turns. All non-magical structures and items are instantly reduced to rubble or useless scrap. Additionally, all flying creatures within one mile of tarrasque take an extra 7 (2d6) thunder and lightning damage, as the world-ending storm turns the skies into a place of devastation.

Retaliate. When struck my a melee attack, the tarrasque can use its reaction to make a tail attack against the creature which made the attack.

The tarrasque 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. The tarrasque regains spent legendary actions at the start of its turn.

Attack. The tarrasque makes one claw attack or one tail attack.

Rampage (Costs 2 Actions. The tarrasque moves up to its full speed, making one claw attack against each creature, object , and structure in its space while it moves.

Deafening Roar (Costs 3 Actions). The world quakes as the tarrasque lets out a mighty roar. Each creature within 60 feet of the tarrasque must succeed on a DC 27 Constitution saving throw or take 26 (4d12) thunder damage and be incapacitated until the end of the tarrasque's next turn.

If the tarrasque's World Ender trait has activated in the last hour, it can use the options below as legendary actions.

Bite. The tarrasque makes one bite attack or it uses swallow.

Earth Splitter (Costs 3 Actions). The tarrasque slams its tail into the ground, causing it to split apart and erupt with magma. Each creature in a 300 foot line that is 20 feet wide and 60 feet tall must succeed on a DC 27 Dexterity saving throw for take 39(6d12) fire damage and be knocked prone.