Combustive Pitch

You spray highly flammable tar from your hands, covering the ground in a 20 ft. square in front of you. Creatures in that square when it appears must make a Dexterity saving throw. If a creature fails, their speed is reduced to 10 ft., and they are considered to be ensnared in the tar. An ensnared creature may use its action to try and free themselves by making a Strength saving throw against your spell save DC. 

If the tar is ignited by fire, all creatures within it must make a Dexterity saving throw, taking 3d6 fire damage on a failed save, and half as much on a successful one.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage caused by the tar when ignited increases by 1d6 for every two slot levels above 2nd.