Oil Slick

You cover the ground in a 10-foot square within range in oil. For the duration, it is difficult terrain.

When the oil appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw.

The oil is flammable. Any 5 foot square of the oil exposed to fire burns away in one round. Each creature who enters the fire or starts it turn there must make a Dexterity saving throw, taking 3d6 fire damage on a failed save, or half as much on a successful one. The fire ignites any flammable objects in the area that aren't being worn or carried.