Voided Strike

You cloak your weapon in energy drawn from the Abyss, allowing you to deal additional damage. You may use a level one spell slot to cast this spell on your weapon. your next attack deals an additional 1d4 necrotic damage, increased to 1d6 if you are hidden. When using higher spell slots, this damage increases by 1d4 or 1d6 for each spell slot level. 

This appears as a cantrip, use the honor system to mark off your spell slots on your own.