Spawn of Zen'Daver (Mythic Trait; Recharges after a Short or Long Rest). If the tarrasque is reduced to 0 hit points, it doesn't die or fall unconscious. Instead it regains 800 hit points and roars, using its Frightful Presence ability, ignoring the immunity of any who have succeeded the save already. It also activates a latent ability within itself. All creatures within 1 mile of the tarrasque cannot fly more than 20 ft. above the ground. This ability considers the roof of a building to be the ground.

Elder of Annihilation. The tarrasque ignores all other creatures' damage immunities and resistances.

Eternal Slumber. When the tarrasque is reduced to 0 hit points, it sinks back down into Zen'Daver's core for the next 1d100 years.

Improved Critical. The Tarrasque's attacks score a critical on 18-20 and when it gets a critical, it rolls the damage dice three times instead of twice.

Immutable Form. The tarrasque cannot be shapechanged by the polymorph spell, or any other similar effect. It also cannot be removed from its home plane by means of plane shift, gate, banishment or any other similar effect. It can however be moved to other places within its home plane by such effects.

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

Limited Magic Immunity. The tarrasque can't be affected or detected by spells of 6th level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.

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 5, the tarrasque is unaffected. On a 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.

Reactive. The tarrasque takes one reaction per turn, rather than one per round.

Regeneration. The tarrasque regenerates 60 hit points at the start of its turn if it has at least 1 hit point remaining.

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

Surefooted. The tarrasque has advantage on saving throws made against effects that would knock it prone.

Multiattack. The tarrasque can use its Frightful Presence. It then makes six attacks: one with its bite, two with its claws, two with its horns, and one with its tail. It can use its Swallow instead of its bite. Alternatively, it makes three Rock attacks.

Bite. Melee Weapon Attack: +24 to hit, reach 10 ft., one target. Hit: 49 (6d12 + 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: +24 to hit, reach 15ft., one target. Hit: 37 (6d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +24 to hit, reach 10ft., one target. Hit: 43 (6d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +24 to hit, reach 20ft., one target. Hit: 31 (6d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 32 Strength saving throw or be knocked prone.

Rock. Ranged Weapon Attack. +24 to hit, range 360/1440 ft., one target. Hit: 49 (6d12 + 10) bludgeoning damage.

Frightful Presence. Each creature of the tarrasque’s choice within 120 feet of it and aware of it must succeed on a DC 20 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 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 tarrasque’s Frightful Presence for the next 24 hours.

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 84 (24d6) acid damage at the start of each of the tarrasque’s turns.

If the tarrasque takes 60 damage or more on a single turn from a creature inside it, the tarrasque must succeed on a DC 16 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.

The tarrasque can take 5 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 tail attack.

Move. The tarrasque moves up to half its speed.

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

If the tarrasque's Mythic Trait is active, it can use the options below as legendary actions.

Rock Throw. The tarrasque makes a rock attack.

Multiattack (Costs 2 Actions). The tarrasque makes 2 claw attacks and a tail attack, or 2 horn attacks, and a tail attack.