Atheras's Blazing Eruption

You lay your hand flat, palm up, and spread the oil and saltpeter across it, gesturing at a point within range. A 15-foot cube centered on the chosen point explodes as fire erupts from the ground. Each creature in that area must make a Dexterity saving throw. On a failed save, a creature takes 4d8 fire damage, or half as much on a successful save. Any creature that enters the area for the first time on it's turn or ends it's turn within the cube takes an additional 4d8 fire damage.

The fire lightly obscures the area within and behind it and lights flammable objects within the space that are not being worn or carried.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d8 for each slot level above 3th.