Tra'quan's Fiery Blast

A ball of fire, 1 foot in diameter, erupts from your outstretched hand, streaking towards a point within range. Upon reaching that point, it explodes, filling a 15-foot cube with flames. All creatures within that cube must make a Dexterity saving throw. On a failed save, the creature takes 6d8 fire damage immediately, and 1d8 at the start of its next turn. On a successful save, the creature takes half as much damage, both immediately and on its next turn. The fire ignites flammable objects in the area that aren't being worn or carried. 

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