Blackfire

You have learned the deepest, darkest magic of the Shadar-kai witches. You are able to summon blackfire, an amalgamation of the burning heat of fire with the necrosis of death itself. A streak of fire-like blackness erupts from your pointing finger to a point you choose within range, and then blossoms with a low roar into an explosion of black colored fire and death. Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 4d6 fire damage and 4d6 necrotic damage on a failed save, or half as much damage of each on a successful one. The blackfire spreads around corners, and ignites and decays all it touches.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for both fire and necrotic damage for each level above 3rd.