Cremation

A thick sheet of flames shoots forth from you. Each creature in a 30ft cone makes a Dexterity saving throw, taking 6d8 Fire damage on a failed save or half as much damage on a successful one.

Flammable objects in the area that aren’t being worn or carried start burning

Using a Higher-Level Spell Slot. The damage increases by 1d8 for each spell slot level above 3