Elitzur's Lightning Lash

You create a lash of lightning energy that strikes at one creature of your choice that you can see within range. The target takes 1d8 lightning damage and must make a Constitution saving throw. On a failed save the targets speed is halved until the end of its next turn. Damage from this spell is non-lethal, if a creature would be brought to zero hit points it instead falls unconscious and is stabilised at 1 hp. 

This spell’s damage increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8).