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

Hellfire. Fire damage dealt by the Scalewing surpasses resistance and immunity to fire damage. If this damage kills a creature, the creature's soul rises from the River Styx as a Lemure in Avernus in 1d4 hours. If the creature isn't revived before then, only a wish spell or killing the, Lemure and casting true resurrection on the creature's original body can restore it to life. Constructs and devils are immune to this effect.

Multiattack. The Scalewing can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 7 (2d6) fire damage.

Claw. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 8) fire damage.

Tail. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage + 10 (2d8) fire damage.

Frightful Presence. Each creature of the Scalewing's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 21 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 Scalewing's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 5–6). The Scalewing uses one of the following breath weapons.

Hellfire Breath. The Scalewing exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 66 (12d10) fire damage on a failed save, or half as much damage on a successful one. 

Weakening Breath. The Scalewing exhales gas in a 60-foot cone. Each creature in that area must succeed on a DC 21 Strength saving throw or have disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws 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.

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 18 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.