Cloud of Rust

You create a 20-foot-radius Sphere of flakes of rust centered on a point within range. The area of the cloud is Heavily Obscured. Each creature that starts its turn in the Sphere must succeed on a Constitution saving throw or stuffer 4d10 Acid damage and take 3 Corrode tokens, or half as much damage and corrode tokens(rounded down) on a success. 

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the damage increases by 1d10 and adds an additional corrode token for each slot level above 5th.