Blackfire

Necrotic black flames wreathes the target. The target must make a Constitution saving throw. It takes 12d8 necrotic damage on a failed save, or half as much damage on a successful one, reducing their hit point maximum by the damage dealt. The target dies if this effect reduces its hit point maximum to 0. Otherwise, the reduction lasts until the target finishes a long rest. On a failed save, the target is poisoned and burns for the spell’s duration. At the end of each of its turns, the target repeats the saving throw. It takes 8d8 necrotic damage on a failed save, and the spell ends on after three successful saves. These necrotic flames can’t be extinguished by nonmagical means.

In addition to the above effects on the target, blackfire can spread rapidly, and any creature who starts or ends their turn within 5 feet of a creature engulfed in blackfire must succeed on a Constitution save or become engulfed itself. If damage from this spell reduces the targets hit points to 0, their body turns to ash and they die. The creature can be restored to life only by means of a true resurrection or a wish spell.