Using your action, you conjure a magic bonfire that fills a 5-foot cube. Any creature in the fire's space must pass a Dexterity save or take 1d8 fire damage. A creature must also make the save when it moves into the bonfire's space or ends its turn there. The bonfire ignites flammable objects in its area that aren't being worn or carried.
The spell's damage die increases by one at levels 5, 11, and 17.