Unholy Smite

You imbue your weapon with unholy energy. The next time you hit a creature with a weapon attack before this spell ends, the attack deals an extra 2d8 necrotic damage to the target. The damage increases by 1d8 if the target is a Celestial. 

Using a Higher-Level Spell Slot. The damage increases by 1d8 for each spell slot level above 1. 

* Which you take immediately after hitting a target with a Melee weapon or an Unarmed Strike