Erupting Lightning

You slam the ground with the palm of your hand, causing pillars of lightning to erupt from the ground beneath your foes. The area of lightning consists of up to seven 5-foot cubes, which you can arrange as you wish. Each cube must have at least one face adjacent to the face of another cube. Each creature in the area must make a Dexterity saving throw. It takes 7d10 lightning damage and falls prone on a failed save, or half as much damage and doesn’t fall prone on a successful one.