Dread Harvest

Your necromancy manifests as a ghostly blade that harvests life and transfers some of its energy to you. All creatures in a 15-foot cone originating from you must make a Constitution saving throw, taking 4d6 necrotic damage on a failed save or half as much on a successful one. 

For each creature that failed its saving throw, your next weapon or spell attack that hits before the end of your next turn deals an additional 2 necrotic damage.

At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the spell damage increases by 1d6, and the attack damage buff increases by 1, for each slot level above 3rd.