As an action, a black beam of necrotic energy forms as a blade in your free hand, and lasts for the duration. You create the blade as a magical dagger, katar, sai, sickle, greatsword, kama, kukri, longsword, scimitar, or shortsword.
The spell ends if you dismiss it on your turn (no action required), if you cast it again, or if you end your turn without the blade in your hand as the blade dissipates.
At higher levels
When you reach 9th level, you can make an attack with the blade as part of the same action to create it.