Apocalyptic Tarrasque

Apocalyptic Empowerment. The Apocalyptic Tarrasque's weapon attacks are considered critical hits on rolls of 18-20, and its damage dice are tripled instead of doubled on a critical hit.

Earthen Slumber. If the Apocalyptic Tarrasque is reduced to 0 hit points and is unable to regenerate for one minute, the Apocalyptic Tarrasque will enter a deep slumber and proceeds to sink into the earth as if by the effects of Earth Glide. During this time, the Apocalyptic Tarrasque is untraceable by magical and nonmagical means. It will then remain asleep for 1d20 +9 years, at which point it will awaken in a random location on the Material Plane. Whether is awakens as the regular Tarrasque or the Apocalyptic Tarrasque is at the DM's Discretion

Freedom of Movement.  Difficult terrain does not impede the Apocalyptic tarrasque's movement, and magical effects can't reduce its speed or cause it to be restrained. The Apocalyptic Tarrasque can expend 5 feet of movement to escape from nonmagical restraints or being grappled.

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

Unique Immortality. The Apocalyptic Tarrasque cannot be killed by conventional means. Spell effects and abilities that instantly kill a creature, such as those of Power Word Kill, will instead drop the Apocalyptic Tarrasque to 0 hitpoints and falls unconsious with no other effects. The only way to kill the Apocalyptic Tarrasque is to drop it to 0 hit points while neutralizing its ability to regenerate and using a Wish to destroy its body. Another Wish can be used to undo this process.

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

Magic Weapons. The Apocalyptic Tarrasque's weapon attacks are considered magical

Reflective Carapace. Any time the Apocalyptic 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 5, the Apocalyptic Tarrasque is unaffected. On a 6, the Apocalyptic Tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the Apocalyptic Tarrasque, turning the caster into the target.

Regeneration. If the Apocalyptic Tarrasque has fewer hipoints remaining than it's hit point maximum, it regains 100 hit points at the start of its next turn, unless it takes 30 acid damage before the start of its next turn.

Siege Monster. The Apocalyptic Tarrasque deals double damage to objects and structures.

Multiattack. The tarrasque can use its Frightful Presence. It then makes five attacks: one with its bite, two with its claws, one with its horns, and one with its tail. It can use its Swallow instead of its bite, and can choose to replace one or both of its claw attacks with a Throw Debris attack.

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.

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Throw Debris. Ranged Weapon Attack:  +19 to hit, reach 120/480ft, one target. Hit: 37 (6d8+10) bludgeoning damage. if the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Earth Rend (Recharges on a roll of 10 on a 1d10) The Apocalyptic Tarrasque slams its claws against the the ground and sends out a massive pressure wave. Each creature on the ground within 120 feet of the Apocalyptic Tarrasque must succeed on a DC 27 Dexterity Saving throw or take 36(8d8) bludgeoning damage plus 36 (8d8) Force damage and be knocked prone. Objects and structures automatically fail the saving throw. If a creature fails by 5 or more, it is also stunned for 1d4 rounds, with the effect ending at the end of its turn. In addition, the ground within 120 feet of the Apocalyptic Tarrasque becomes difficult terrain. 

Frightful Presence. Each creature of the Apocalyptic Tarrasque's choice within 120 feet of it and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Apocalyptic Tarrasque 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 the Apocalyptic Tarrasque's Frightful Presence for the next 24 hours.

Roar of Disruption (Recharge 6) All creatures within a 120 foot cone originating from the Apocalyptic Tarrasque must succeed on a DC 25 Constitution saving throw or take 72 (16d8) thunder damage and become stunned until the end of their next turn. In addition, any spellcaster who fails the save must also make a DC 27 saving throw using their spellcasting attribute or be unable to cast or concentrate on spells for 1d4 rounds.

Sky Rend (Recharges on a roll of 10 on a 1d10) The Apocalyptic Tarrasque slams its claws together and sends out a massive shock-wave . Each creature in the air within 120 feet of the Apocalyptic Tarrasque must succeed on a DC 25 Dexterity Saving throw or take 36(8d8) thunder damage plus 36 (8d8) Force damage and be knocked prone. if a creature fails by 5 or more, it is also stunned until the end of their next turn., with the effect ending at the end of its turn. In addition, the air within 120 feet of the Apocalyptic Tarrasque becomes difficult terrain for 1d4 rounds, which ends at the start of the Apocalyptic Tarrasque's turn 

Swallow. The Apocalyptic 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 Apocalyptic Tarrasque, and it takes 56 (16d6) acid damage at the start of each of the Apocalyptic Tarrasque's turns.

If the Apocalyptic tarrasque takes 60 damage or more on a single turn from a creature inside it, the Apocalyptic tarrasque 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 the Apocalyptic Tarrasque. If the Apocalyptic Tarrasque is knocked unconscious, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

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

Attack. The Apocalyptic Tarrasque makes one claw attack or tail attack.

Move. The Apocalyptic Tarrasque moves up to its speed.

Chomp (Costs 2 Actions). The Apocalyptic Tarrasque makes one bite attack or uses its Swallow.

World Shatter( Costs 3 Actions). The Apocalyptic Tarrasque uses both Sky Rend and Earth Rend (If both are available)