Tar Pit

You create a 20-foot-radius sphere of mysterious dark goo on the ground centered on a point within range. The goo spreads around corners. Any creature that ends its turn in the goo takes and 4d6 acid damage. Any creature caught in the radius of the spell must make a strength saving throw or be unable to move for their turn. A successful throw allows movement through the goo as difficult terrain.

The goo has 50 hit points and only takes damage from cold attacks. The goo has an armor class of 8.