Armageddon

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

Magic Resistance. The beast has advantage on saving throws against spells and other magical effects.

Heart Of The Tarrasque. Every turn the beast regains 20 health the only time it doesn’t regenerate is if it took psychic damage or is stunned.

Reflective Carapace. Any time the beast 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 2, the beast is unaffected. On a 3 to 4, the tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the beast, turning the caster into the target. On a 5 to 6 the beast is uneffected, and the effect is reflected back at 2X damage 

Siege Monster. The beast deals quadruple damage to objects and structures.

Multiattack. The beast can use its Frightful Presence. It then makes eight attacks: two with its bite, two with its claws, two with its horns, and two with its tail. It can use its Swallow instead of one of its bite.

Bite. Melee Weapon Attack: +21 to hit, reach 10 ft., one target. Hit: (4d20 + 17) piercing damage. If the target is a creature, it is grappled (escape DC 26). Until this grapple ends, the target is restrained, and the beast can't bite another target.

Claw. Melee Weapon Attack: +21 to hit, reach 15ft., one target. Hit: (5d10+ 13) slashing damage.

Horns. Melee Weapon Attack: +21 to hit, reach 10ft., one target. Hit: (5d20+ 2) piercing damage.

Tail. Melee Weapon Attack: +21 to hit, reach 20ft., one target. Hit: (10d12 + 18) bludgeoning damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of the beast’s choice within 200 feet of it and aware of it must succeed on a DC 24 wisdom saving throw or become frightened for 5 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the beast 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 beast's Frightful Presence for the next 5 hours.

Swallow. The beast 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 beast, and it takes (16d8) acid damage at the start of each of the beast's turns.

If the beast takes 75 damage or more on a single turn from a creature inside it, the beast must succeed on a DC 26 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 45 feet of movement, exiting prone.

Godly Breath (Recharge 8). The beast uses any other dragon's breath weapon in a 150 foot cone doing (5d20+64) (any damage type).

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

Attack. The beast makes one claw attack and tail attack.

Move. The beast moves its whole speed.

Chomp. The beast makes one bite attack or uses its Swallow.