Clockwork Tarrasque

Legendary Resistance (6/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.

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

Multiattack. The tarrasque makes one Bite attack and three other attacks, using Claw or Tail in any combination.

Bite. Melee Attack Roll: +19, reach 15 ft. Hit: 36 (4d12 + 10) Piercing damage, and the target has the Grappled condition (escape DC 20). Until the grapple ends, the target has the Restrained condition and can’t teleport.

Claw. Melee Attack Roll: +19, reach 15 ft. Hit: 28 (4d8 + 10) Slashing damage.

Tail. Melee Attack Roll: +19, reach 30 ft. Hit: 23 (3d8 + 10) Bludgeoning damage. If the target is a Huge or smaller creature, it has the Prone condition.

Concentrated Cannon. (Recharge 5-6) Dexterity Saving Throw: DC 26, creatures in a 15 ft. Wide 150 foot long line. The targets take 133 (38d6) lightning damage, or half as much on a successful save. 

Reconstruct. The tarrasque heals 6d12 hp. If the tarrasque has taken lightning damage since its last turn, it heals an additional 4d12 hp. 

Legendary Action Uses: 3. Immediately after another creature’s turn, the tarrasque can expend a use to take one of the following actions. The tarrasque regains all expended uses at the start of each of its turns.

Onslaught. The tarrasque moves up to half its Speed, and it makes one Claw or Tail attack.

World-Shaking Movement. The tarrasque moves up to its Speed. At the end of this movement, the tarrasque creates an instantaneous shock wave in a 60-foot Emanation originating from itself. Creatures in that area lose Concentration and, if Medium or smaller, have the Prone condition. The tarrasque can’t take this action again until the start of its next turn.