|“||A black key. Handle in the shape of a skeleton. It can open a door to Death's library.||”|
The Key to Death is a special black key with a skeleton-shaped handle that has the power to open a door to Death's library.
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.