Obsidian Dagger

The cursed Obsidian Dagger has a sickly dark energy and longs to be whole again. It uses the pain it inflicts to fuel the transformation.
At DM's discretion the blade gains the following effects in order.
-The dagger does an additional 1d4 slashing damage.
-The dagger does an additional 1d6 necrotic damage.
-At the start of the turn the user can take 1d20 damage to double all damage they deals this turn.