Key to Death

The Key to Death is a special black key with a skeleton-shapped handle that has the power turn any ordinary door into a portal to Death's library.

Background
According to Sergei, the Men of Letters purchued the Key to Death, a key that can grant access to Death's library, storing it after, somewhere.

Season 15
In Last Call, Sergei tricks Castiel and Eileen by worsening Sam's condition so he could exchange the cure for the Key to Death. However, Castiel had predicted Sergei's betrayal and had Bobby to watch over his niece as leverage against Sergei, forcing him to heal to Sam.

Appearances

 * Season 15
 * Last Call (mentioned)

Trivia

 * It's unknown if the Key to Death was purchued by the American branch of the Men of Letter or the British one. By extension, it's unknown if the Key to Death is somewhere stored in the bunker.