Chaos Tarrasque

Amphibious. The tarrasque can breathe air and water.

Earth Glide. The tarrasque can burrow through nonmagical, unworked earth and stone. While doing so, the tarrasque doesn't disturb the material it moves through.

Innate Spellcasting. The chaos tarrasque’s spellcasting ability is Charisma (spell save DC 18). it can innately cast the following spells, requiring no Verbal, Somatic, or Material components:

1/day each: Enthrall, Hellish Rebuke (see "Reactions" below)

Keen Smell. The tarrasque has advantage on Wisdom (Perception) checks that rely on smell.

Legendary Resistance (3/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.

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

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.

Regeneration. No form of attack can suppress the tarrasque’s regeneration, it regenerates even if disintegrated or slain by a death effect. If the tarrasque falls below 0 hit points, it rises from death in 1d10 rounds with 50 hit points. It can be banished or otherwise transported as a means to save a region, but the method to truly kill it has yet to be discovered.

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

Tough Immunity. The tarrasque is immune to all diseases, and any effect that would reduce its maximum hit points.

Multiattack Options. The chaos tarrasque has the following multiattack options:

The tarrasque 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 flame tail. It can use its Swallow instead of its bite.

The tarrasque can use its Frightful Presence. It then uses its Acid Breath (if available) and makes one flame tail attack.

The tarrasque can use its Frightful Presence. It then uses its Flame Tail Breath (if available) and makes two claw attacks.

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage plus 9 (2d8) acid 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 15 ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 32 (4d10 + 10) piercing damage. If the target is a creature, it must make a DC 20 Constitution saving throw or be stunned until the start of the tarrasque’s next turn.

Flame Tail. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 28 (4d8 + 10) bludgeoning damage plus 11 (2d10) fire damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of the tarrasque's choice within 120 feet of it and aware of it must succeed on a DC 18 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.

Acid Breath (Recharge 5–6). The tarrasque exhales acid in a 90-foot line that is 10 feet wide from its mouth. Each creature in that line must make a DC 24 Dexterity saving throw, taking 72 (16d8) acid damage on a failed save, or half as much damage on a successful one.

Flame Tail Breath (Recharge 5–6). The tarrasque exhales fire in a 90-foot cone from its tail head. Each creature in that area must make a DC 24 Dexterity saving throw, taking 71 (13d10) fire damage on a failed save, or half as much damage on a successful one.

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. The tarrasque has three stomachs. Any creature or object swallowed takes 6 rounds to pass through each stomach in order.

First Stomach. A maze of sword like teeth and powerful muscles to shred and grind everything into paste. Everything in the first stomach takes 36 (8d8) slashing damage plus 36 (8d8) bludgeoning damage at the start of each of the tarrasque’s turns.

Second Stomach. A toxic pool of magic eating acid. Spells are unaffected by the acid, but magical items have a 50% chance of being destroyed at the start of each of the tarrasque’s turns. Everything in the second stomach takes 63 (18d6) acid damage at the start of each of the tarrasque’s turns.

Third Stomach. A seemingly endless twisting tunnel of extreme heat. Everything in the third stomach takes 70 (20d6) fire 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, 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.

Hellish Rebuke. A creature that damages the tarrasque within 60 feet is momentarily surrounded by hellish flames. The tarrasque can use its reaction to force a creature to make a DC 18 Dexterity saving throw. It takes 2d10 fire damage on a failed save, or half as much damage on a successful one.

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.

Move. The tarrasque moves up to half its speed.

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