Avatar of Onachus

Legendary Resistance (6/Day). If the Avatar fails a saving throw, it can choose to succeed instead.

Magic Resistance. The Avatar has Advantage on saving throws against spells and other magical effects.

Reflective Carapace. If the Avatar is targeted by a Magic Missile spell or a spell that requires a ranged attack roll, roll 1d6. On a 1–5, the Avatar is unaffected. On a 6, the Avatar is unaffected and reflects the spell, turning the caster into the target.

Shed the Mortal Vessel.  The Avatar heals to full Health and can use its Shed abilities.  

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

Multiattack. The Avatar 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 or cast spells.

Shed Bite. Melee Attack Roll: +19, reach 15 ft. Hit: 36 (4d12 + 10) Radiant 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 or cast spells.

Claw. Melee Attack Roll: +19, reach 15 ft. Hit: 28 (4d8 + 10) Slashing damage and the target cannot benefit from healing effects until the start of the Avatar's next turn.

Shed Claw. Melee Attack Roll: +19, reach 15 ft. Hit: 28 (4d8 + 10) Radiant damage and the target cannot benefit from healing effects until the start of the Avatar's next turn.

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.

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

Thunderous Bellow (Recharge 5–6). Constitution Saving Throw: DC 27, each creature and each object that isn’t being worn or carried in a 150-foot radius Sphere. Failure: 98 (15d12) Thunder damage, and the target has the Deafened and Frightened conditions until the end of its next turn. Success: Half damage only.

World-Shaking Movement. The Avatar moves up to its Speed. At the end of this movement, the Avatar creates an instantaneous shock wave in a 60-foot radius originating from itself. Creatures in that area must make a DC 27 Constitution saving throw or lose Concentration and, if Medium or smaller, have the Prone condition. The Avatar can’t take this action again for two more turns.

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

Chomp. The Avatar makes a bite attack.

Flurry. The Avatar makes two Shed claw attacks.  Only usable when Shed.