Obsidian Butterfly Knife

Made of razor-sharp obsidian, this finely-balanced blade glows faintly with deep purple light. 

You gain a +1 bonus to attack and damage rolls made with this dagger. 

You can use an action to cause the dagger’s inner light to brighten, glowing like the corona of an eclipsed sun. This glow lasts for 1 minute or until you use it to deal damage to a creature. That creature makes a DC 12 Constitution saving throw or it takes 3d6 necrotic damage and is unable to regain hit points for 1 minute. If this damage reduces the target to 0 hit points, the target immediately dies and 1d4 rounds later its body explodes into a swarm of obsidian butterflies that completely eviscerate the corpse, leaving only the heart behind. The dagger can’t be used this way again until it is exposed to a new sunrise.