Acid Fog

You create a 20-foot-radius sphere of fog 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 11 miles per hour) disperses it. Creatures that enter the fog cloud for the first time move through it as though it were difficult terrain; in addition, at the beginning of your turn each round, the fog deals 5d6 points of acid damage to each creature and object within it. 

At Higher Levels. When you cast this spell using a spell slot of 7th level or higher, the radius of the fog increases by 20 feet for each slot level above 6th.