Creature Sense. The Tarrasque is aware of the presence of creatures within 5 miles of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each one’s Intelligence score, but can’t sense anything else about it. A creature protected by a mind blank spell, a nondetection spell, or similar magic can’t be perceived in this manner.

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

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.

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

Telepathic Hub. The tarrasque can use its telepathy to initiate and maintain telepathic conversations with up to ten creatures at a time. The tarrasque can let those creatures telepathically hear each other while connected in this way.

Unusual Nature. The tarrasque doesn’t require air or sleep. 

Multiattack. The tarrasque can use its Frightful Presence. It then makes six attacks: one with its bite, two with its claws, one with its horns, one with its tail, and one with its tentacle It can use its Swallow instead of its bite, and use its tentacle instead of any of its claw attacks. It then uses its Spellcasting.

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 tarrasque can’t bite another target.

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.

Tentacle. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 28 (4d8 + 10) bludgeoning damage If the target is a Huge or smaller creature, it is grappled (escape DC 15) and takes 9 (1d8 + 5) psychic damage at the start of each of its turns until the grapple ends. The tarrasque can have up to four targets grappled at a time by this ability.

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. At the end of each of the tarrasque's turns, all swollowed creatures are infested with illithid tadpoles.

While infested, the creature takes 16 (3d10) psychic damage at the start of each of its turns. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself after it succeeds on three of these saves. If the creature is targeted by magic that ends a curse or restores 40 hit points or more, the tadpoles infesting the creature are killed instantly, ending the effect on the creature.

If a Humanoid is reduced to 0 hit points while infested, the creature is stable but remains unconscious for 6d12 hours. When the period of unconsciousness ends, the creature transforms into a mind flayer (see the Monster Manual) with all its hit points. Casting a wish spell on the unconscious creature rids it of the infestation and prevents it from turning into a mind flayer.

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.

Spellcasting (Psionics). The tarrasque casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

At will: detect thoughts, levitate

3/day: modify memory

1/day each: dominate monster, plane shift (self only)

Mind Blast (Recharge 5–6). Creatures of the tarrasque’s choice within 120 feet of it must succeed on a DC 18 Intelligence saving throw or take 60 (10d10) psychic damage and be stunned for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. 

Psychic Link. The tarrasque targets one incapacitated creature it senses with its Creature Sense trait and establishes a psychic link with the target. Until the link ends, the tarrasque can perceive everything the target senses. The target becomes aware that something is linked to its mind once it is no longer incapacitated, and the tarrasque can terminate the link at any time (no action required). The target can use an action on its turn to attempt to break the link, doing so with a successful DC 18 Charisma saving throw. On a successful save, the target takes 10 (3d6) psychic damage. The link also ends if the target and the tarrasque are more than 5 miles apart. The tarrasque can form psychic links with up to ten creatures at a time.

Sense Thoughts. The tarrasque targets a creature with which it has a psychic link. The tarrasque gains insight into the target’s emotional state and foremost thoughts (including worries, loves, and hates).

The tarrasque 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 tarrasque regains spent legendary actions at the start of its turn.

Attack. The tarrasque makes one claw attack, tail attack, or tentacle attack.

Move. The tarrasque moves up to half its speed.

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

Break Concentration. The tarrasque targets one creature within 120 feet of it with which it has a psychic link. The tarrasque breaks the creature’s concentration on a spell it has cast. The creature also takes 2 (1d4) psychic damage per level of the spell.

Psychic Pulse. The tarrasque targets one creature within 120 feet of it with which it has a psychic link. The target and enemies of the tarrasque within 30 feet of target take 10 (3d6) psychic damage.

Sever Psychic Link. The tarrasque targets one creature within 120 feet of it with which it has a psychic link. The tarrasque ends the link, causing the creature to have disadvantage on all ability checks, attack rolls, and saving throws until the end of the creature’s next turn.