Uncle Joe's Shellshock

This spell was first granted to Ploqwat, a Tortle Hexblade warlock. 

A spectral turtle shell made of lighting appears on your back if you do not have one. if you do, lightning energy courses around it. The next successful  melee attack on you within the next minute shocks your attacker with an arc of magical lighting. The attacker takes 1d8 lightning damage, and it can't take reactions until the start of its next turn.

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