Ancient Hellwing

Bonemelt Hellfire. Fire and acid damage dealt by the Hellwing 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.

Devil's Sight. Magical darkness doesn't impede the hellwings darkvision.

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

Multiattack. The hellwing makes two attacks: one with its bite and one with its bone melt spray if it has recharged. While flying, it also makes two attacks with its claws.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one creature. Hit: 26 (3d10 + 10) piercing damage plus 14 (4d6) acid damage plus 14 (4d6) fire damage.

Claws. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 21 (2d10 + 10) slashing damage plus 5 (1d10) fire damage.

Bone Melt spray (Recharge 5-6). The hellwing exhales a 60-foot long, 15 foot wide line of sticky, acidic hellfire. Each creature in that area must make a DC 26 Dexterity saving throw, taking 55 (10d10) fire damage plus 35 (10d6) acid damage on a failed save, or half as much damage on a successful one. The fire remains on the ground forever until removed by magic. Until that time has expired, each creature that starts or ends it's turn in the fire takes 55 (10d10) fire damage plus 35 (10d6) acid damage. A creature damaged by the fire, until it is doused by magic, takes 35 (10d6) fire damage plus 28 (8d6) acid damage at the start of each of it's turns for one minute until it is scraped off using an action. This effect does not stack.