Alistair's Roaring Pyre

You cause a source of magical or nonmagical fire to flare up into a roaring inferno. Until the spell ends, the flames fill a 5 foot cube. Any creatures within 5 feet of the cube must make a Dexterity saving throw or take 3d6 fire damage. As a bonus action you may cause the flames to spread 5 feet in any direction. The flames ignite any flammable objects in its area that aren't being worn or carried.