Astral Entity. The Astral Tarrasque can't leave the Astral Plane, nor can it be banished or otherwise transported out of the Astral Plane.

Legendary Resistance (3/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.

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 to 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.

Voidborn. The astral tarrasque has its own gravity, and does not require air to live.

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.

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage and 6 (1d10) fire 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 and 4 (1d6) poison damage.

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage and 4 (1d8) cold damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage and 6 (3d4) lightning damage. If the target is a creature, it must succeed on a DC 20 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 (8d12) 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 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 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.

Frightful Presence. Each creature of the tarrasque's choice within 120ft 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 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.

Planetary Roar. All creatures within 120ft must succeed a DC 19 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 12 hours.

Earth Shaking Movement. As a bonus action after moving at least 10 feet on the ground, the astral tarrasque can send a shock wave through the ground in a 120-foot-radius circle centered on itself. That area becomes difficult terrain for 1 minute. Each creature on the ground that is concentrating must succeed on a DC 25 Constitution saving throw or the creature's concentration is broken.

Elemental Endurance. When the astral tarrasque takes damage that includes cold, fire, force, lightning or force damage, the astral tarrasque can use it's reaction to roll 2d12 + 10, subtracting the total from the attacks damage.

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 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.

Planetary Roar (Costs 2 Actions). Each creature of the tarrasque's choice within 120 feet of it and aware of it must succeed on a DC 19 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.

Booming Roar (Costs 2 Actions). Each creature within 60ft must make a DC 18 Charisma saving throw taking 21 (2d12 + 10) thunder damage on a failed save, or half on a success. Each frightened creature takes an additional 1d12 thunder damage.

Elemental Fury (Costs 3 Actions). The astral tarrasque deals twice as much fire, cold, poison and lightning damage until the start of it's next turn.