You touch a creature and afflict them with necrotic energy that saps their strength away. Make a melee spell attack against a target. On a hit the creature takes 1d8 necrotic damage. The first time that creature deals damage after making an attack roll, roll 1d8. Subtract the number on the die from the damage dealt. 

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