Alchemical Boiler

You magically create an erupting fountain of boiling oil centered on a point within range. The oil spills out to a radius of 20ft and the area becomes difficult terrain. Any creature that enters the area or starts its turn there must make a dexterity saving throw. On a failed save, the creature takes 6d6 fire damage and falls prone. On a successful save, a creature takes half as much damage and doesn't fall prone. The pool of oil disperses at the end of your next turn, and the spell ends.

Using a Higher-Level Spell Slot. The damage increases by 1d6, and the spells duration increases by 1 round for each spell slot level above 3.