Unholy Weapon

You imbue a weapon you touch with unholy power. Until the spell ends, the weapon shimmers with a dark, pulsating purplish aura. In addition, weapon attacks made with it deal an extra 2d8 necrotic damage on a hit. If the weapon isn't already a magic weapon, it becomes one for the duration.

As a bonus action on your turn, you can dismiss this spell and cause the weapon to emit a burst of necrotic energy. Each creature of your choice that you can see within 30 feet of you must make a Constitution saving throw. On a failed save, a creature takes 4d8 necrotic damage, and gains 1 level of exhaustion. On a successful save, a creature takes half as much damage.