Death's ring.
Death's ring is a ring that Death wears. Unlike all the other Horsemen, Death doesn't need it to channel his powers, and isn't devastatingly lost without it.
About
Proven in the episode Appointment In Samarra, whoever wears the ring of Death must take his place. In the episode, Dean takes the place of Death for 24 hours in order to retrieve Sam's soul from Lucifer's Cage.