Chaos Tarrasque

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.

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 3, the tarrasque is unaffected. On a 4-6 the chaos 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.

Anti-Magic Field. For 300 ft around this creature, no magic can be cast.

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

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. It can use its Swallow instead of its bite.

Bite. Melee Weapon Attack: +20 to hit, reach 10 ft., one target. Hit: 40 (4d12 + 14) 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: +20 to hit, reach 15ft., one target. Hit: 30 (4d8 + 12) slashing damage.

Horns. Melee Weapon Attack: +20 to hit, reach 10ft., one target. Hit: 40 (4d10 + 18) piercing damage.

Tail. Melee Weapon Attack: +20 to hit, reach 20ft., one target. Hit: 30 (4d6 + 16) bludgeoning 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 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.

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 60 (17d6) acid damage at the start of each of the tarrasque's turns.

If the tarrasque takes 100 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 [condition]prone[/conditions.

Chaos Breath Weapon. During an attack action, a chaos tarrasque can, instead of its bite and tail, it can instead use its breath weapon. This breath weapon is a 100ft cone in front of the chaos tarrasque. The target or targets, have to make a dex, con, and str saving throw or take 2d20 fire, 2d20 cold, 2d20 acid, 2d20 lightning, and 2d20 necrotic damage. If the target is reduced to 0 hit points from this breath weapon, they immediately die. Once this feature is used, roll a d6. on a 3-6 this ability recharges.

Reactive Attacks. When this creature is attacked, it can use its reaction to do 2 claws and 1 bite attack against the creature.

The tarrasque can take 5 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 two claw attacks, bite attacks, and tail attacks.

Move. The tarrasque moves up to half of its speeds each.

Defend. (costs 2 actions) The tarrasque can add 10 temporary points to its ac. 

Recharge. The chaos tarrasque can roll to recharge its chaos breath weapon twice.