Charge. If the tarrasque moves at least 20 feet in a staight line towards a target and then hits it with an attack with its horns, the target takes an additional 22 (4d10) bludgeoning damage. If the target is a creature, it must make a DC 20 strength saving throw or be flung up to 3d6 × 10 feet away, landing prone.

Colossal. The tarrasque is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks.
In addition, whenever the tarrasque makes a weapon attack, regardless of whether the attack hits or misses, each creature within 20 feet of the attack's target other than the tarrasque itself must succeed on a DC 20 Dexterity saving throw or takes half the attack's damage.

Empowered Attacks. The tarrasque's attacks count as adamantine and as magical for the purpose of overcoming resistances and immunities to nonmagical attacks and damage.

Earthbinding Aura. While within 120 feet of the tarrasque, creatures have their flying speed halved and can't fly higher than 60 feet above ground level, as the earth itself pulls them downwards towards the tarrasque's hungry maw. If a creature enters this area while flying at a higher altitude, it starts descending at a rate of 60 feet per round

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

Immutable Existence. The tarrasque is immune to any spell or effect that would alter its form or send it to another plane of existence.

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 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.

Regeneration. The tarrasque regains 50 hit points at the start of each of its turns. This regeneration can not be suppressed by any means short of a wish spell. The tarrasque dies only if it starts its turn with 0 HP and does not regenerate.

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

Trample. The tarrasque ignores difficult terrain and can enter spaces occupied by other creatures. The first time on a turn the tarrasque willingly moves into the space of another creature, that creature must succeed on a DC 20 Dexterity saving throw or takes 26 (4d12) bludgeoning damage and is knocked prone. 
In addition, the tarrasque automatically deals 26 (4d12) damage to objects and structures with an AC of 18 or less in its path, and any space the tarrasque moves through becomes difficult terrain.

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 20 ft., one target. Hit: 36 (4d12 + 10) piercing 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 30ft., one target. Hit: 28 (4d8 + 10) slashing damage. If the target is a creature, it must succeed on a DC 20 strength saving throw or be flung up to 3d6 × 10 feet away from the tarrasque, landing prone.

House-Sized Boulder. Ranged Weapon Attack: +19 to hit, range 240/600 feet, one target. Hit: 88 (12d12+10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Horns. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 50ft., 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 tarrasque’s choice within 120 feet 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.

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 (16d6) 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.

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 melee attack.

Move. The tarrasque moves up to half its speed.

Boulder. (Costs 2 Actions). The tarrasque makes one attack with its House-Sized Boulder.

Earth-Shaking Stomp (Costs 2 Actions, recharge 5, 6). The tarrasque stomps, causing the earth within 120 feet of it to shake violentlly. The Area becomes difficult terrain until cleared and each grounded creature in this area must make a DC 20 dexterity saving throw. On a failure, it takes 52 (8d12) bludgeoning damage and is knocked prone, on a success, it takes half as much damage and is not knocked prone.
In addition, 1d6 fissures open in random locations. Each fissure is 1d10 × 10 feet deep, 10 feet wide, and up to 120 feet long. A creature standing on a spot where a fissure opens falls in if it fails its saving throw. A creature that successfully saves moves with the fissure's edge as it opens.
Objects and structures take the damage automatically, and a fissure that opens beneath a structure causes it to automatically collapse