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

Magic Resistance. The Ancient Tarrasque has advantage on saving throws against spells and other magical effects.

Regeneration. The Ancient Tarrasque regains 20 hit points at the start of its turn if it has at least 1 hit point.

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

Siege Monster. The Ancient Tarrasque deals double damage to objects and structures.

Antimagic Field. A 90-foot-radius invisible sphere of antimagic surrounds the Ancient Tarrasque. This area is divorced from the magical energy that suffuses the multiverse. Within the sphere, spells can't be cast, summoned creatures disappear, and even magic items become mundane.

Spells and other magical effects, except those created by an artifact or a deity, are suppressed inside the sphere. A slot expended to cast a suppressed spell is consumed. While an effect is suppressed, it doesn't function, but the time it spends suppressed counts against its duration.

Multiattack. The Ancient 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, its Acid Breath or its Poison Breath instead of its bite.

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage plus 10 (3d6) poison damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the Ancient Tarrasque can’t bite another target. the grappled creature takes  7 (3d4) acid damage at the start of each of the Ancient Tarrasque’s turns.

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

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

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., 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 Ancient 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 Ancient 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 Ancient Tarrasque’s Frightful Presence for the next 24 hours.

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

If the Ancient Tarrasque takes 60 damage or more on a single turn from a creature inside it, the Ancient 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 Ancient Tarrasque. If the Ancient 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.

Acid Breath (Recharge 4). The Ancient Tarrasque exhales acid in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 67 (15d8) acid damage on a failed save, or half as much damage on a successful one.

Poison Breath (Recharge 4). The Ancient Tarrasque exhales poisonous gas in a 90-­-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 77 (22d6) poison damage on a failed save, or half as much damage on a successful one.

The Ancient 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 attack or tail attack.

Move. The tarrasque moves up to half its speed.

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