Adult Tarrasque

Legendary Resistance (5/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.

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.

Tunneler. The tarrasque can burrow through solid rock at half its burrowing speed and leaves no tunnel in its wake.

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

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

Enraged Hunger (Recharges after a Short or Long Rest). If the tarrasque would be reduced to 0 hit points, its current hit point total instead resets to 500 hit points,  and it regains any expended uses of Legendary Resistance. Additionally, the tarrasque can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating the tarrasque after its Enraged Hunger activates.

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 one with its tail or tail spikes. It can use its Swallow instead of its bite.

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the tarrasque can’t bite another target.

Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 30ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Tail Spikes. Ranged Weapon Attack: +19 to hit, range 100/200 ft., one target. Hit: 20 (4d4+ 10) piercing damage.

Frightful Presence. Each creature of the tarrasque’s choice within 120 feet of it and aware of it must succeed on a DC 25 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. This effect is magical. 

Swallow. The tarrasque makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite’s 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 30 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. If a creature or object is within this stomach for 1 min, it is sent to the tarrasque third stomach. Any creature or object inside this stomach is disintegrated by heat. Dealing 105 (10d10+50) fire damage that bypasses immunities.

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

Attack. The tarrasque makes one claw attack, tail attack or tail spikes attack.

Move. The tarrasque moves up to half its speed.

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

If the tarrasque's Enraged Hunger trait has activated in the last hour, he can use the options below as legendary actions.

Acid Spit (2 actions). The tarrasque spits acid from its second stomach up to 70 feet. The spit splashes out in a 15ft radius from a point, creatures and objects in that area need to make a DC 27 Dexterity saving throw or take Hit: 32 (4d10+ 10) acid damage. Creatures inside the stomach of the tarrasque gets caught in the attack and also needs to make the save and are spit out onto the point of the attack. Creatures take have damage on a successful save.

Enraged (3 actions). The Enraged tarrasque flails around. Anything with 30-foot-radius centered on the tarrasque needs to make a DC 27 Dexterity saving throw or take Hit: 36 (4d12 + 10) bludgeoning damage and be knocked prone. On a successful save the creature takes half damage and isn't knocked prone.