Legendary Resistance (4/Day). If the creature fails a saving throw, it can choose to succeed instead.

Reflective Carapace. Any time the creature 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 creature is unaffected. On a 6, the creature is unaffected, and the effect is reflected back at the caster as though it originated from the creature, turning the caster into the target.

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

Innate Spellcasting (3/Day). The creature can innately cast divine word (spell save DC 26). Charisma is their spell casting ability.

Limited Magic Immunity. The creature will not be affected by spells of 6th level or lower, unless it wants to be.

Magic Weapons. All of the creature's attacks count as magical.

Regeneration. The creature regains 30 hit points at the start of its turn.

Multiattack. The creature 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 10 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 creature can't bite another target.

Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 28 (4d8 + 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 creature'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 creature's Frightful Presence for the next 24 hours.

Swallow. The creature 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 creature, and it takes 56 (16d6) acid damage at the start of each of the creature's turns.

If the creature takes 60 damage or more on a single turn from a creature inside it, the creature 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 creature can take 5 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 creature regains spent legendary actions at the start of its turn.

Attack. The creature makes one claw attack or tail attack.

Move. The creature moves up to half its speed.

Chomp (Costs 2 Actions). The creature makes one bite attack or uses its Swallow.

Wing Attack (Costs 2 Actions). The creature beats its tattered wings. Each creature within 10 ft. of the creature must succeed on a DC 21 Dexterity saving throw or take 24 (4d6 + 9) bludgeoning damage and be knocked prone. After beating its wings this way, the creature can fly up to half its flying speed.

Breath Attack (Costs 2 Actions).The creature uses one of its different colored heads to spit a powerful breath. This can do acid, lightning, poison, fire, or cold damage. All of them except for its poison breath require a dexterity saving throw, poison is constitution. The DC is 27. The range is a 90 feet cone, and deals 88 (16d10) lightning damage.