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

Sheer Force. All damage that the tarrasque deals is magical for the purposes of overcoming resistance and immunity to non-magical damage.

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.

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

Regeneration. When the tarrasque begins its turn with 1 hit point or more and isn't incapacitated, it regains 30 hit points.

Colossal. A creature that is Large or smaller can enter the tarrasque’s space, treating the tarrasque's space as difficult terrain. As soon as a Large or smaller creature enters the tarrasque’s space, the tarrasque immediately makes a Stomp attack against it (no action required). Space occupied by such a creature is not considered difficult terrain for the tarrasque. A creature that’s within the tarrasque’s space can target the softer underbelly. Attacks and spells targeted at the tarrasque that are made this way treat the tarrasque’s AC as 20, and ignore the Reflective Carapace ability.

Rampage (half/297 or less hit points). As soon as the tarrasque's hit points reach 297 or fewer, the tarrasque’s Earthbind ability ends early and is disabled. The tarrasque gains a 10 ft. bonus to its walking, climbing, and swimming speed, and an additional two attacks when it takes the Attack action; one with its claws, and one with either its tail sweep, tail spike, or stomp. The tarrasque also gains the use of its Leap action. Rampage doesn't end until the tarrasque takes a short or long rest.

 Additionally, if the tarrasque were to drop to 0 hit points while Rampage is active, it must make a DC 20 Constitution saving throw. On a success, the tarrasque is dropped to 1 hit point instead.

Multiattack. The tarrasque can use its Frightful Presence. It then makes five attacks: two with its claws, one with its bite or swallow, one with its tail sweep or tail spike, and one with its stomp 

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature that is Huge or smaller, 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 if the target is a creature that is Huge or smaller and has already taken damage from the tarrasque's claw attack this turn, it must also make a DC 27 Strength saving throw or be knocked 10 ft. away from the tarrasque and to prone.

Stomp. The tarrasque targets one point that's within its space. Each creature within a 5ft. radius of this point that's Large or smaller must make a DC 18 Dexterity saving throw or take 28 (4d8+10) bludgeoning damage.

Tail Sweep. The tarrasque swings its tail in a 25 ft. cone originating from itself. Each creature within this area that's Huge or smaller must make a DC 22 Dexterity saving throw or takr 32 (4d10+10) bludgeoning damage and be knocked 20 ft. away from the tarrasque and to prone. 

Frightful Presence. Each creature of the tarrasque's choice within 120 feet of it and aware of it must succeed on a DC 18 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.

Tail Spike. Ranged Weapon Attack: +10 to hit, range (150/600 ft.), one target. Hit: 27 (4d12+1) piercing damage.

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 52 (15d6) acid damage at the start of each of the tarrasque's turns.

If the tarrasque takes 58 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.

Barrage. The tarrasque makes one Tail Spike attack against any number of creatures of its choice within a 40 ft. sphere of an area that it can see.

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, tail sweep, or tail spike attack.
Move. The tarrasque moves up to half its speed.
Chomp (Costs 2 Actions). The tarrasque makes one bite attack or uses its Swallow.