Blight Aura

You emit a deathly aura of blight that saps the life of those around you in a 10 ft. Emanation. Each creature that starts its turn within the aura must make a Constitution saving throw, taking 3d6 Necrotic damage on a failure, or half as much damage on a success. Creatures with the Plant type have Disadvantage on this saving throw. Foliage, vegetation, and plants that are not creatures don't make a saving throw; they simply wither and die.

Using a Higher-Level Spell Slot. The Necrotic damage increases by 1d6 per level of spell slot above 3.