Amaterasu

You create an area of 5ft of extremely hot, black fire. Each creature in the area of the spell must make a Dexterity saving throw, on a failed save, the target takes 3d6 then 1d6 over the next 10 rounds, or half as much on a successful save. The caster may dispell any area of the flames as long as the caster can see it. Unlike normal fire, this flame spreads slow enough to remove the effected area. For example, the victim could cut the straps to his breastplate to remove it before it spreads, or remove an arm before it spreads.