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

Awake The World Eater (Recharge on a long rest). When the Tarrasque is reduced to 0 hit points or killed, it instead regains 300 hit points and its legendary resistances, its annihilating breath instantly recharges, and the tarrasque gains access to both its mythic actions and regeneration ability. Award your party and additional 155,000 XP for defeating the Tarrasque after this ability activates.

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.

Magic Sense. While it has a sense of smell, the tarrasque can sense magic (as if using a detect magic spell) within 240 ft of it.

Terror Unmatched. The tarrasque, even to the bravest and most hardy, is a terrifying foe. Even creatures that have no mind and feel no fear quake at the sight of it. Immunity to the frightened condition instead only grants advantage on saving throws against being frightened by the Tarrasque.

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

Magic Weapons. The tarrasque's weapon attacks are magical.

Regeneration (Awake the World Eater). At the start of each of the Tarrasque's turns, it regains 40 hit points. Nothing short of a wish spell can prevent this regeneration. The Tarrasque dies only if it starts its turn with 0 hit points and does not regenerate.

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 either a tail swipe or a tail slam. It can use its Swallow instead of its bite, and it can replace on or both of tis claw attacks with spine shots.

Bite. The Tarrasque targets up to eight Large or smaller creatures within 20 ft of it that are within 5 ft of eachother. Each target must make a DC 27 dexterity saving throw, or else take 36 (4d12+10) piercing damage. If the target is a creature, it is grappled (escape DC 27). Until this grapple ends, the target is restrained, and the tarrasque can’t bite any more targets.

Alternatively, the tarrasque can target four creatures that are Huge, or one that is Gargantuan, that are within 20 ft of it and 5 ft of each other, making an attack roll (1d20+19) against each target instead of forcing a saving throw for the same effect.

Claw. The Tarrasque swipes its huge claws at six Large or smaller creatures that are within 30 ft of it and within 5 ft of eachother. Each target must make a DC 27 dexterity saving throw, taking 28 (4d8+10) slashing damage on a failed save or half that on a succesful one.

Alternatively, the tarrasque can target three creatures that are Huge, or one that is Gargantuan, that are within 20 ft of it and 5 ft of each other, making an attack roll (1d20+19) against each target instead of forcing a saving throw for the same effect.

Horns. The Tarrasque thrusts its gigantic horns forward. Each creature in a 20 foot wide, 20 foot long line must make a DC 27 Strength saving throw. On a failed save, the target takes 32 (4d10+10) piercing damage and is restrained (escape DC 27). On a succesful save, a target takes half that damage and is not restrained.

Tail Swipe. The Tarrasque swings its tail in an arch, each creature in a 60 ft by 40 ft cube centered on the Tarrasque (excluding the Tarrasque) must make a DC 27 Dexterity saving throw. On a failed save a target takes 24 (4d6+10) bludgeoning damage, and if a target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Tail Slam. The Tarrasque slams its tail into the ground, causing an earthquake to shake the surrounding area. Each creature in a 40 ft long, 15 ft wide line must make a DC 27 dexterity saving throw or else take 24 (4d6+10) bludgeoning damage. Then each creature within 100 ft of the line (excluding the Tarrasque) must make a DC 27 Dexterity saving throw or be knocked prone. If an effected creature is concentrating on a spell, it must make a DC 27 constitution saving throw, breaking its concentration on a fail. 

The tremor deals 50 bludgeoning damage to any structure in contact with the ground in the area. If a structure drops to 0 hit points, it collapses and potentially damages nearby creatures. A creature within half the distance of a structure's height must make a DC 20 Dexterity saving throw. On a failed save, the creature takes 5d6 bludgeoning damage, is knocked prone, and is buried in the rubble, requiring a DC 20 Strength (athletics) check as an action to escape. On a successful save, the creature takes half as much damage and doesn't fall prone or become buried.

 Annihilating Breath (Recharge 6). The Tarrasque inhales, bending the weave and forming a hideous spacial hole in its throat that sucks in an condenses pure magic. After a build up of power, the Tarrasque releases a destructive cone of pure force. Each creature in a 300 foot cone must make a DC 27 Constitution saving throw. On a failed save, the target takes 155 (11d20+40) force damage and if the target is reduced to 0 hit points, it is disintegrated. A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust. The creature can be restored to life only by means of a true resurrection or a wish spell. The breath also automatically disintegrates a Large or smaller nonmagical object or a creation of magical force. If the target is a Huge or larger object or creation of force, the breath disintegrates a 50-foot- cube portion of it. A magic item is unaffected by this spell.

Frightful Presence. Each creature of the tarrasque’s choice within 240 feet of it and aware of it must succeed on a DC 24 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 targets anything that is huge or smaller it is grappling with its bite. A target must make a DC 27 strength saving throw. On a failed save the target takes 36 (4d12+10) piercing 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 25 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.

Spine Shot.  The tarrasque can target up to six large or smaller creatures or objects within 300 ft. of it, provided they are within 5 feet of each other. It fires one of its spines from its back towards the targets. Each target must make a DC 27 dexterity saving throw, taking 4d8+10 piercing damage on a failed save or half that on a success.

Alternatively, the Tarrasque can target three huge or smaller creature, making an attack roll (1d20+19) instead of forcing a saving throw for the same effect. This attack has a normal range of 120 ft. and a long range of 300 ft.

Earth-Shaking Movement. As a bonus action after moving at least 10 feet on the ground, the tarrasque can send a shock wave through the ground in a 240-foot-radius circle centered on itself. That area becomes difficult terrain for 1 minute. Each creature on the ground that is concentrating must succeed on a DC 27 Constitution saving throw or the creature’s concentration is broken.

The shock wave deals 100 thunder damage to all structures in contact with the ground in the area. If a creature is near a structure that collapses, the creature might be buried; a creature within half the distance of the structure’s height must make a DC 27 Dexterity saving throw. On a failed save, the creature takes 17 (5d6) bludgeoning damage, is knocked prone, and is trapped in the rubble. A trapped creature is restrained requiring a successful DC 20 Strength (athletics) check as an action to escape. Another creature within 5 feet of the buried creature can use its action to clear rubble and grant advantage on the check. If three creatures use their actions in this way, the check is an automatic success. On a successful save, the creature takes half as much damage and doesn’t fall prone or become trapped.

The tarrasque can take 4 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.

Move. The tarrasque moves up to half its speed.

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

Annihilating Breath (3 actions). The tarrasque recharges and uses its annihilating breath.

Stomp (2 actions). The tarrasque brings its mighty weight and slams it onto the ground. Each creature within 100 ft of the Tarrasque must make a DC 27 dexterity saving throw, taking 24 (4d6+10) bludgeoning damage an a failed save or half as much on a success. The area is also subject to the tarrasque's earth-shaking movement.