Adult Tarrasque Dragon

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

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

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

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

Multiattack. The dragon 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: +18 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage plus 7 (2d6) fire damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the dragon can’t bite another target.

Claw. Melee Weapon Attack: +18 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +18 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +18 to hit, reach 20ft., one target. 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.

Frightful Presence. Each creature of the dragon’s choice that is within 120 feet of the dragon 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 dragon’s Frightful Presence for the next 24 hours.

Swallow. The dragon 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 dragon, and it takes 56 (16d6) acid damage at the start of each of the dragon's turns.

If the dragon takes 60 damage or more on a single turn from a creature inside it, the dragon 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 dragon. If the dragon dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Fire Breath (Recharge 5–6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

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

Detect. The dragon makes a Wisdom (Perception) check.

Attack. The dragon makes one claw attack or tail attack.

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

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 22 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.