Dark Blade

You sprinkle powdered obsidian over your weapon and it begins to hum with dark energy, for the duration the weapon becomes magical if it was not before and for the next 10 minutes deals an additional 4d8 necrotic damage. in addition to this each time a creature is struck with the weapon it must succeed on a Constitution saving throw or gain one level of exhaustion, if they succeed they are not subjected to this affect unless struck again in which case they are required to make another save.