Divine Immolation

A bright streak flashes from your pointing finger to a point you choose within range, where it slowly ignites the air itself with phlogiston. Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 4d6 radiant damage on a failed save, or half as much damage on a successful one. Every target that takes damage takes the same amount of damage again at the start of your next two turns unless it uses an action to put out the flames on its clothing, hair, and other accouterments. Flammable objects in the area that aren't being worn or carried are not ignited. 

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