Tarrasque

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.

The tarrasque has advantage on saving throws against spells and other magical effects.

3 legendary actions; one at a time at the end of another creature’s turn, regaining spent uses at the start of its turn

3/day. If the tarrasque fails a saving throw, it can choose to succeed instead.

The monster deals double damage to objects and structures.

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.

Melee Weapon Attack: +19 to hit, reach 10 ft.. 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.

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

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

Melee Weapon Attack: +19 to hit, reach 20 ft.. Hit: 24 (4d6 + 10) bludgeoning damage.
If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Each creature of the tarrasque's choice that is within 120 ft. of the tarrasque and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute.
A creature can repeat the saving throw at the end of each of its turns, 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.

The tarrasque makes one Bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target 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. A tarrasque can have only one creature swallowed at a time.
If the tarrasque takes 30 damage or more on a single turn from the swallowed creature, the tarrasque must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate the creature, which falls 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 15 feet of movement, exiting prone.

The tarrasque makes one claw attack or tail attack.

The tarrasque moves up to half its speed.

Costs 2 legendary actions. The tarrasque makes one bite attack or uses its Swallow.