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

Brimstone Aura. Creatures that begin their turns within 10 feet of the dragon must succeed on a DC 19 Constitution saving throw or take 28 (8d6) fire damage. Creatures that touch the dragon take an immediate 7 (2d6) fire damage and have disadvantage on the next saving throw they make against this ability within the next minute.

Semi-Functional Wings. The dragon's wings are pitted with holes, which do not allow it to fly properly. The dragon must exert massive effort to take flight; as such, the dragon can only use its fly speed if it takes the Dash action on its turn. The dragon can still move up to twice its fly speed while Dashing. While falling, the dragon can glide up to one foot in any direction for every two feet fallen.

Brimstone Wings. While flying, the dragon's wings create a vortex of fire underneath them. A 40-foot come extends from the bottom of the dragon and moves with it. Creatures that enter this cone for the first time or end their turns in it take 28 (8d6) fire damage. This cone disappears if the dragon ends its turn without taking the Dash action. 

Brimstone Lungs. The dragon can breathe both air and lava.

Multiattack. The dragon can make two attacks as part of the Attack action: one with its bite and one with its claws.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 24 (3d10 + 8) piercing damage plus 7 (2d6) fire damage.

Claw. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 18 (3d6 + 8) slashing damage plus 7 (2d6) fire damage.

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 21 (3d8 + 8) bludgeoning damage plus 7 (2d6) fire damage.

Brimstone 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. Creatures within the cone must also make a DC 21 Constitution saving throw; on a failure, they take an extra 7 (2d6) fire damage whenever they would take fire damage for the next minute. Creatures that fail the save can re-roll it at the end of each of their turns.

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.

Tail Attack. The dragon makes a tail attack.

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 15 (2d6 + 8) bludgeoning damage plus 7 (2d6) fire damage and be knocked prone.