Multiattack

The tarrasque attacks with its bite claw horns and tail. It can use Swallow instead of its bite. If its bloodied it also recharges and then uses Radiant Breath.

Bite

Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 42 (5d12 + 10) piercing damage. If the target is a creature it is grappled (escape DC 27). Until this grapple ends the target is restrained and the tarrasque can't bite a different creature.

Claw

Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 32 (5d8 + 10) slashing damage.

Horns

Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 37 (5d10 + 10) piercing damage.

Tail

Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 27 (5d6 + 10) bludgeoning damage. If the target is a Huge or smaller creature it falls prone.

Swallow

The tarrasque makes a bite attack against a Large or smaller creature it is grappling. If the attack hits the target is swallowed and the grapple ends. A swallowed creature has total cover from attacks from outside the tarrasque it is blinded and restrained and it takes 35 (10d6) acid damage and 35 (10d6) bludgeoning damage at the start of each of the tarrasques turns.

If a swallowed creature deals 70 or more damage to the tarrasque in a single turn, or if the tarrasque dies, the tarrasque vomits up all swallowed creatures

Radiant Breath (Recharge 5-6)

The tarrasque exhales radiant energy in a 90-foot cone. Each creature in that area makes a DC 27 Constitution saving throw taking 105 (30d6) radiant damage on a failed save or half damage on a success.