ArchTarrasque

Legendary Resistance (9/Day). If the Archtarrasque fails a saving throw, it can choose to succeed instead.

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

Greater Reflective Carapace. Any time the Archtarrasque 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 Archtarrasque is unaffected. On a 3-6, the Archtarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the Archtarrasque, turning the caster into the target.

Greater Siege Monster. The Archtarrasque deals quintuple damage to objects and structures.

Multiattack. The Archtarrasque can use its Frightful Presence. It then makes twelve attacks: two with its bite, two with its thunderous roar, four with its claws, two with its horns, and two with its tail. It can use its Swallow instead of its bite.

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 62 (8d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 30). Until this grapple ends, the target is restrained, and the Archtarrasque can’t bite another target.

Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 46 (8d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 54 (8d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 38 (8d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 30 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of the Archtarrasque’s choice within 120 feet of it and aware of it must succeed on a DC 25 Wisdom saving throw or become frightened for 4 minutes. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Archtarrasque 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 Archtarrasque’s Frightful Presence for the next 4 hours.

Swallow. The Archtarrasque makes one bite attack against a gargantuan 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 Archtarrasque, and it takes 112 (32d6) acid damage at the start of each of the Archtarrasque’s turns.

If the Archtarrasque takes 1200 damage or more on a single turn from a creature inside it, the Archtarrasque must succeed on a DC 10 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 Archtarrasque. If the Archtarrasque dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Call Offspring (1/Week). The Archtarrasque lets out a bellowing roar and summons (1d8) Tarrasques.

Thunderous Roar.  the Archtarrasque roars at the top of its lungs, releasing such powerful sound waves that all creatures in a 150ft cone originating from the beast's mouth must make a constitution saving throw or take 45 (8d10) thunder damage on a failed save or half as much on a successful one.

The tarrasque can take 9 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 or tail attack.

Move. The tarrasque moves up to half its speed.

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