Ash Cloud

You create a 20-foot-radius sphere of smoke centered on a point within range. The sphere spreads around corners, and its area is heavily obscured. It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it.

As an action, you can ignite the smoke. Each creature in the smoke must make a Dexterity save. On fail, targets take 4d4 fire damage and knocked prone. On save, the targets take half damage and are not knocked prone. 

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the radius of the smoke increases by 20 feet and the damage increases by d4 for each slot level above 1st.