Tar pit

Tar covers the ground in a 20-foot square centered on a point within range and turns it into difficult terrain for the duration.

When the tar appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there takes 3d6 fire damage and must also succeed on a Dexterity saving throw or fall prone. 

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.