Creature Sense. The Illithirrasque 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.

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 Illithirrasque's attacks count as Adamantine and as magical for the purpose of overcoming resistance or immunity to nonmagical attacks.

Earthbinding Aura. Within 120 feet of the Illithirrasque 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 Illithirrasque'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 Illithirrasque fails a saving throw, it can choose to succeed instead.

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

• at will: detect thoughts, levitate, telekinesis
• 3/day each: dominate monster, wall of force

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

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

Regeneration. The Illithirrasque 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 Illithirrasque dies only if it starts its turn with 0 HP and does not regenerate.

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

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

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 Illithirrasque can use its Frightful Presence. It then makes six attacks: two with its claws, three with its tentacles, and one with its tail.

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 in a random direction away from the Illithirrasque.

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.

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.

Tentacle. Melee Weapon Attack: +19 to hit, reach 60 ft., one target. Hit: 32 (4d10 + 10) bludgeoning damage. If the target is a creature, it is grappled (escape DC 20) and is restrained until the grapple ends. The Illithirrasque can have up to 12 targets grappled at a time.

Frightful Presence. Each creature of the Illithirrasque’s choice within 120 feet of it and aware of it must succeed on a DC 22 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 Illithirrasque 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 Illithirrasque's Frightful Presence for the next 24 hours.

Brain-Extracting Swallow. The Illithirrasque makes one tentacle attack against each target it is grappling. If the attack hits, the target takes the attack's damage, is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained and has total cover against attacks and other effects outside the Illithirrasque. In addition, it takes 36 (8d8) piercing damage and 36 (8d8) psychic damage and has its intelligence score reduced by 1d4 at the start of each of the Illithirrasque’s turns.

If the attack's initial damage or the damage taken at the start of the Illithirrasque's turn reduces a creature to 0 HP or its intelligence score is reduced to 0, the Illithirrasque kills the target by extracting, consuming or digesting its brain.

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

Mind-Blasting Roar (recharge 5, 6). The Illithirrasque magically lets out a massive, deafening roar fueled by both rage and psychic energy, which penetrates the minds of other creatures. Creatures of the Illithirrasque's choice within 120 feet of it must succeed on a DC 22 Intelligence saving throw or take 88 (16d10) psychic damage and be stunned and deafened 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.

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

Attack. The Illithirrasque makes one melee attack.

Move. The tarrasque moves up to half its speed.

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

Earth-Shaking Stomp (Costs 2 Actions, recharge 5, 6). The Illithirrasque 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 100 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.

Consume Brains (Costs 3 Actions). The Illithirrasque uses its Brain-Extracting Swallow.