Key to Death's Library
From Super-wiki
The Key to Death, was acquired sometime ago by the Men of Letters. Also known as the Black Key it has a handle in the shape of a skeleton, and can open a door to Death's Library.
Episodes
15.07 Last Call
When Castiel calls on the shaman Sergei to help with Sam's wound, Sergei puts Sam's life in danger, demanding that he will only heal Sam if Castiel and Eileen find the Black Key and give it to him. Castiel has however arranged for Bobby to find Sergei's niece, Anna, and he threatens to harm her if Sergei does not heal Sam.
