Pestilence's ring.
Pestilence's ring is a ring that helps the Green Horseman, Pestilence, channel and focus his powers. Sam and Dean obtained the ring after Castiel cut off Pestilence's fingers using Ruby's knife. Without it, Pestilence cannot exist on the physical plane.
When combined with the other three horsemen's rings, it can open up a portal to send Lucifer back to his cage again.