Fire Storm

A storm of fire appears within range. The area of the storm consists of up to ten 10-foot Cubes, which you arrange as you like. Each Cube [Area of Effect] must be contiguous with at least one other Cube [Area of Effect]. Each creature in the area makes a Dexterity saving throw, taking 7d10 Fire damage on a failed save or half as much damage on a successful one.

Flammable objects in the area that aren't being worn or carried start burning.