As you use the sword form, it starts to glow a bright red, you can use this to perform a zerosum discharge, causing a large explosion to shoot out from the end of the blade, anything hit by it takes 25d20 fire damage

The fire spreads around corners. It 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 increases by 1d6 for each slot level above 3rd.