Voltaic Touch

Lightning springs from your hand to deliver a shock to a creature you try to touch. Make a melee spell attack against the target. You have advantage on the attack roll if the target is wearing armour made of metal. On a hit, the target takes 1d8 lightning damage + your spellcasting ability modifier, and it can't take reactions until the start of its next turn. If the target doesn't succeed on a Constitution DC 8 + proficiency bonus + your spellcasting modifier the target is stunned 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.