Blackfire

You engulf a creature you can see, within range, with an aura of black fire. At the start of each of its turns, the target must make a Constitution saving throw. On a failed save, its Constitution score is temporarily reduced by 1d4 + 1 . If its Constitution score drops to 0 before the spell ends, the target dies instantly. Any reduction in the target’s Constitution score is reversed after the target takes a long rest. The spell ends if the target makes three successful saving throws.