The Rings and the desire for blood

"The Cage you sprung Lucifer from? It's still down there. And maybe, just maybe, you can shove his ass back in.... And here's the big secret, Lucifer himself doesn't even know; the key to the cage? It's out there. Actually it's keys, plural. Four keys, well, four rings. From the Horsemen. You get 'em all, you got the Cage."
                        —Gabriel's message for Sam and Dean
         Lucifer was trapped inside his cage for a long time, and when Sam released him, by accident, he didn't plan on going back inside. After he killed little brother Gabriel, another one of the archangels, Dean and Sam got a message from him. In his message Gabriel, also known as the trickster tells them about the key to the cage.



The rings like magnets, are attracted to each other, so when they are put together they stay together. Once an incantation is said, the cage will be opened. That is the easy part of the operation, though. The hard part, is having Lucifer jump into it. He will not go freely, but if the vessel can take control long enough, they can jump in with Lucifer inside, which is near impossible, and has never been heard of before.