Valda's Wildfire

You create a magical fire that fills ten 5-foot cubes on ground, which you can arrange as you wish within range. Each cube you place must have a face adjacent to the face of another cube. On each of your turns after you cast this spell, you can use a bonus action to expand the area of the fire to up to ten additional 5-foot cubes that each have a face adjacent to the
face of another cube of fire. The fire damages objects in the area and ignites flammable objects that aren't being worn or carried. When a creature moves into the area of fire for the first time or starts its turn there, it must make a Dexterity saving throw. It takes 8d8 fire damage on a failed save, or half as much on a successful save.