Your weapon becomes cloaked in an inky, black nimbus. You make a single melee weapon attack against a creature you can see in range. If the attack hits, the target suffers the normal effects of the attack, and that dark energy flows into the wound and leaves him pale, weak, and shaking. You gain  3 points of damage reduction for the next attack you receive or until the end of your next turn.

At 5th level, the spell deals an extra 1d6 necrotic damage and damage reduction increase by 2 points. Damage dice increase by one again at levels 11 and 17 while damage reduction increase to 7 at levels 11 and 9 at levels 17.