Bigby’s Flesh Mortification

You hold the piece of mortified flesh in your hand and place a drop of acid on it while concentrating on your target. Instantly their flesh begins to rot and turn putrid. The target takes 3d8 necrotic damage, constitution save for half. On each subsequent turn you can concentrate on the same target or switch to another within sight, target must make a constitution saving throw each time they are targeted. 
Changing targets requires the caster to cut a piece of flesh from themselves dealing 1d4 points of slashing damage.