Void Ash

You point to an area within range and small bits of black fire and ash fill that 15ft cubed area anyone who starts or ends there turn in that space must make a Dexterity saving throw. A creature takes 6d4 Fire damage on a failed save and half as much on a successsful one, and the falling ash makes the area unbreathable. Any creature in the area that needs to breath has to hold there breath, but if they get hit while in the effected area must make a Con save equal to 5 + damage dealt if they fail, they lose 1 minute worth of air. (see suffocation rules PHB 183)