Voltaic Sphere

A large 15 by 15 foot sphere of light blue electric energy appears at a point within range. At the end of your turn, all creatures inside the sphere must make a constitution saving throw or take 2d4 lightning damage, taking half as much on a successful save. As a bonus action on each of your turns, you may move the sphere 35 feet. The second time it would deal damage, it makes the same roll but instead creatures take 3d4 lightning damage on a failed save. The third time it would deal damage, the sphere explodes and dissipates, dealing 4d4 lightning damage on a successful constitution save, or half as much on a successful one. You may only have one of these spheres out at any given point in time.