Astral Tarrasquenought

Anti magic cone: the tarrasguenought's eye creates an area of anti-magic, as in the antimagic field spell, in a 150-ft cone. At the start of each of its turns, it decides which way the cone faces. The cone doesn't function while the eye is closed or while the tarrasquenought is blinded.

Tarrasque dungeon plane: anything the tarrasquenought swallows is transported to a Demi plane that can be entered by no other means except a wish spell or the tarrasquenought's bite and dungeon visit. The Demi plane resembles an open field that exists endlessly, however it is only 1000 feet in diameter, with a 100 ft high ceiling. The Demi plane contains remnants of past meals, an astral dreadnought, and a tarrasque. The tarrasquenought can't be harmed from within the demiplane. If the tarrasquenought dies, the demiplane disappears, and everything inside it appears around the tarrasquenought's corpse. The demiplane is otherwise indestructible.

Legendary resistance (6/day): if the tarrasquenought fails a saving throw, it can choose to succeed instead.

Magic resistance: The tarrasquenought has advantage on saving throws against spells and other magical effects.

Reflective carapace: anytime the tarrasquenought is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1-5, the tarrasquenought is unaffected. On a 6, the tarrasquenought is unaffected, and the effect is reflected back at the caster as though it originated from the tarrasquenought, turning the caster into the target.

Sever silver cord: if the tarrasquenought scores a critical hit against a creature traveling by means of the astral projection spell, the tarrasquenought can cut the target's silver cord instead of dealing damage.

Siege monster: the tarrasquenought deals double damage to objects and structures.

Unusual nature: the tarrasquenought doesn't require air, food, drink, or sleep.

Multi attack: the tarrasquenought 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.

Bite: melee weapon attack: +35 to hit, reach 15 ft., one target. Hit: 36 (4d12+10) piercing damage plus 36 (5d10+9) force damage. If the target is a Huge or smaller creature and this damage reduces it to 0 hit points or it is incapacitated, the tarrasquenought swallows it. The swallowed target, along with everything it is wearing and carrying, appears in an unoccupied space on the floor of the tarrasque dungeon plane.

Claw: melee weapon attack: +35 to hit, reach 20 ft., one target. Hit: 19 (3d6+9) force damage + 28 (4d8+10) slashing damage.

Horn: melee weapon attack: +19 to hit, reach 10 ft., one target. Hit: 32 (4d10+10) piercing damage.

Tail: melee weapon attack: +19 to hit, reach 20 ft., 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.
Frightful Presence: Each creature of the tarrasquenought’s choice within 120 ft 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 tarrasquenought 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 tarrasquenought’s Frightful Presence for the next 24 hours.

The tarrasquenought can take 6 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 tarrasquenought regains spent legendary actions at the start of its turn.

Attack: the tarrasquenought makes one claw attack or tail attack.

Move: the tarrasquenought moves up to half its walking speed.

Tarrasqueportation (costs 2 Actions): One Huge or smaller creature that the tarrasquenought can see within 60 feet of it must succeed on a DC 19 Charisma saving throw or be teleported to an unoccupied space on the floor of the Tarrasque Dungeon Plane.

Psychic Projection (Costs 3 Actions): Each creature within 60 feet of the tarrasquenought must make a DC 19 Wisdom saving throw, taking 26 (4d10+4) psychic damage on a failed save, or half as much damage on a successful one.