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.
Characteristics
The Key to Death was acquired at some point 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, 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.
15.17 Unity
In order to speak with Billie, Sam and Castiel search through the lore about the key before looking through the bunker's artifacts. Castiel eventually locates the key inside of a box with a red skull on it. When Sam reads the Latin inscription on the box, the key glows and a doorway appears on the nearby wall, leaving behind a keyhole. When Sam turns the key, the door to Death's Library opens for him.
When Sam returns, he removes the key from the lock.