Key to Death's Library
The Men of Letters, some time ago, procured a very important item. A black key. Handle in the shape of a skeleton. It can open a door to Death's library.
When Castiel calls on the shaman Sergei to help with Sam's wound, Sergei puts Sam's life in danger, then states that he will only heal Sam if Castiel and Eileen find the Black Key and give it to him. However, Castiel has arranged for Bobby to monitor Sergei's niece, Anna, and he threatens to have her harmed if Sergei does not heal Sam.