Bleed

From Kobold Press's "Deep Magic"

Crackling energy coats the blade of one weapon you are carrying that deals slashing damage. Until the spell ends, when you hit a creature with the weapon, the weapon deals an extra 1d4 necrotic damage and the creature must make a Constitution saving throw. On a failed save, the creature suffers a bleeding wound. Each time you hit a creature with this weapon while it suffers from a bleeding wound, your weapon deals an extra 1 necrotic damage for each time you have previously hit the creature with this weapon (to a maximum of 10 necrotic damage). 

Any creature can take an action to stanch the bleeding wound by succeeding on a Wisdom (Medicine) check against your spell save DC. The wound also closes if the target receives magical healing. This spell has no effect on undead or constructs.