Mythic Tarrasque

Damage Threshold. The Tarrasque has damage threshold 25. If a creature or effect does damage multiple times in a single action, add the damage together to determine whether the damage threshold is exceeded.

Elder of Annihilation. The Tarrasque's damage cannot be reduced by resistance or immunity. If the Tarrasque attacks a force object, that object is destroyed.

Juggernaut. No form of terrain costs the Tarrasque extra movement to pass through. No effect can reduce the Tarrasque's speed. The Tarrasque may use its burrow speed to move through solid objects as long as they are no tougher than stone.

Legendary Reactions. The Tarrasque may spend a legendary action to take an additional reaction, but may not take two reactions in one turn or take a legendary action on the same turn.

Legendary Recovery. The Tarrasque may spend a legendary action to remove a non-disabling condition or effect, and two legendary actions to remove a disabling condition. If it lacks sufficient legendary actions, it may spend 25 hp per action it lacks.

Legendary Regeneration. If the Tarrasque starts its turn with unspent legendary actions, it heals by 25 hp per unspent action. If the Tarrasque ends its turn without taking an action, it heals by 100 hp.

Legendary Resistance. If the Tarrasque fails a save, it may spend two legendary actions to succeed. If it lacks sufficient legendary actions, it may spend 25 hp per action it lacks.

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.

Save Substitution. If the Tarrasque fails a save, it may retry that save as a Constitution save.

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

Supernatural Dread. Any creature that comes within 150' of the Tarrasque and can see or hear it must make a Wisdom save (DC 22) or be frightened. An affected creature may repeat the save at the end of each of its turns, and it ends automatically after a minute outside of the area. Any creature that succeeds on a save is immune for the next 24 hours.

Unkillable. The Tarrasque cannot die (but see Arena of Earth).

Arena of Earth (Recharges after ten minutes). If the Tarrasque would be killed or reduced to 0 hit points, its current hit point total instead resets to 600, and vast walls of rock rise from the ground, forming a pyramidal arena. The inside of the walls has a 100' radius, rising to a point 100' above the Tarrasque, and the walls are 10' thick (treat as wall of stone: AC 15, 30 hp/inch). Two fissures (as the mythic action below) appear within the area. For the next ten minutes, or until the Tarrasque is defeated, the earth within the arena quakes; at the start of the Tarrasque's turn, creatures in the area other than the Tarrasque must make a Constitution save (DC 22) or be knocked prone and lose concentration. For the duration, the Tarrasque may use mythic actions, and gains one extra legendary action per turn. If the Tarrasque is reduced to zero hit points within the arena, a fissure appears under it, it falls in, and is entombed, as if affected by the burial effect of the imprisonment spell. Award a party an additional 155,000 XP (310,000 XP total) for defeating the Tarrasque within the arena of earth.

Multiattack. The Tarrasque uses one of Bite, Horns, and Swallow, then either Tail Sweep or two Claw attacks.

Bite. Melee Attack Roll: +19, reach 15 ft. Hit: 54 (8d10 + 10) Piercing damage, and the target has the Grappled condition (escape DC 27). 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.

Horns. Melee Attack Roll: +19, reach 15 ft. Hit: 36 (4d12 + 10) Piercing damage. If the Tarrasque moved 20+ feet straight toward it immediately before the hit, the target takes an extra 26 (4d12) Piercing damage and has the Prone condition.

Inexorable (only if above 600 HP). The Tarrasque heals by 100.

Swallow. Strength Saving Throw: DC 27, one Large or smaller creature Grappled by the Tarrasque (it can have up to six creatures swallowed at a time). Failure: The target is swallowed, and the Grappled condition ends. A swallowed creature has the Blinded and Restrained conditions and can’t teleport, 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 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, each of which falls in a space within 10 feet of the Tarrasque and has the Prone condition. If the Tarrasque dies, any swallowed creature no longer has the Restrained condition and can escape from the corpse using 20 feet of movement, exiting Prone.

Tail Sweep. Dexterity Saving Throw: DC 27, creatures in a 30' cone. Failure: 28 (4d8 + 10) Bludgeoning 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 Emanation. Failure: 39 (6d12) Thunder damage, and the target has the Stunned condition until the end of its next turn, and the Deafened condition for the next minute. Success: Half damage and Deafened until the end of its next turn.

Trample. Dexterity Saving Throw: DC 27, each creature or object whose space the Tarrasque moves into before the end of its turn, but no more than once per turn. Failure: 19 (2d8 + 10) Slashing damage. If the target is a Huge or smaller creature, it has the Prone condition.

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.

Earthbind. Strength Saving Throw: DC 27, one creature within 600'. Failure: The target loses the ability to fly for one minute, possibly causing it to fall. Affected creatures may repeat the save at the end of their turns, ending the effect on success.

Maul. The Tarrasque moves up to half its speed and uses Claw.

Onslaught (Recharge at start of turn). The Tarrasque uses one of Bite, Horns, Swallow, or Tail Swipe. If it uses Horns, it may also move up to its speed before doing so.

If the Tarrasque is within its Arena of Earth, it may take the following legendary actions, but may only use each once before the start of its next turn.

Close Fissure. One fissure within the arena closes. Any creature in the fissure at the time it closes suffers 14 (4d6) bludgeoning damage and must make a Dexterity save (DC 27) or be restrained; whether or not it is restrained, it lacks line of sight on any other creature until it exits the fissure. A restrained creature may spend its action to make an acrobatics or athletics check (DC 20); a creature that is not restrained may climb towards the surface but treats the fissure as difficult terrain in addition to any climbing penalties.

Eruption. Dexterity Saving Throw: DC 27, each creature in a cylinder 10' wide and 40' tall within the arena. Failure: 21 (6d6) fire damage. Success: Half damage. The cylinder remains until the start of the Tarrasque's next turn; any creature that enters the cylinder or ends its turn in the cylinder is subject to the effects above.

Open Fissure. a fissure 10' wide, 40' deep, and extending the length of the arena appears. Creatures in the area of the fissure when it opens must make a Dexterity save (DC 27) or fall in, taking 14  (4d6) falling damage. Structures in the area of the fissure are destroyed.