Brooj's Burning Bog

As you crush the moss and charcoal in your hand, the ground in a 15-foot radius centered on a point you can see within range becomes overgrown with swamp vegetation, and the air surrounding it becomes obtrusively hot. The area becomes difficult terrain for the duration. When a creature moves into or starts it's turn within the area, it must make a Constitution saving throw or take 3d8 fire damage. The creature takes half as much damage on a successful save.

 When you cast this spell using a spell slot of 4th level or higher, the radius of the effected area increases by 5-feet per spell level spent.