Volatile Strike

As an action you make a melee spell attack, channeling elemental lightning into the shape of a hammer, electrocuting a single target. On a successful melee hit, you deal 4d8 Lightning damage. Additionally, a creature successfully hit by this spell must make a Constitution saving throw or become stunned until the end of its next turn. Lastly, so long as a target is stunned (but not unconscious) by this spell, any creature who comes within 5 feet of the target automatically takes lightning damage equal to your spell casting modifier. 

At Higher Levels: When cast using a spell slot of 5th level or higher, the aftershock damage increases to equal your spell casting modifier+1d4 per spell slot level. When cast at higher spell slots, you can increase the initial damage by an additional 1d8 per spell slot above 4th.