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

Immutable Form. The tarrasque is immune to any spell or effect that would alter its form.

Keen Smell. The tarrasque has advantage on Wisdom (Perception) checks that rely on smell.

Multiattack. The tarrasque makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 24 (3d10 + 5) piercing damage.

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 16 (2d8 + 5) slashing damage.

Swallow. The tarrasque makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the tarrasque, and it takes 21 (6d6) acid damage at the start of each of the tarrasque's turns. If the tarrasque takes 30 damage or more on a single turn from a creature inside it, the tarrasque must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate the swallowed creature, which falls 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 using 15 feet of movement, exiting prone.

Tail Swipe. The tarrasque makes a tail attack against each creature within 10 feet of it. Each target must succeed on a DC 15 Dexterity saving throw or take 16 (2d8 + 5) bludgeoning damage and be knocked prone.

Frightful Presence. Each creature of the tarrasque's choice within 120 feet of it and aware of it must succeed on a DC 15 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.