Without me, you got zero shot at killing Lucifer. Sorry. But... you can trap him. The cage you sprung Lucifer from, it's still down there, and maybe, just maybe, you can shove his ass back in. Not that it'll be easy. You got to get the cage open, trick my bro back into it, and oh yeah, avoid Michael and the God Squad. But hey - details, right? And here's the big secret, Lucifer himself doesn't even know. But 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.
Castiel described the Seals as locks on a door that held Lucifer prisoner. On God's command the great archangel named Michael bound his younger brother, Lucifer, with 600 seals. According to Anna only 66 had to be broken for him to be freed from his confinement.
In 1972, Azazel possessed a priest at St. Mary's Convent, and killed eight nuns in the chapel. This enabled him to speak to the imprisoned Lucifer. Lucifer tells Azazel that Lilith is needed to break The 66 Seals that hold him captive in the Cage and that he needs to find a very special child.4.22 Lucifer Rising
The first Seal to the cage is broken by Dean while in Hell:
And it is written that the first seal shall be broken when a righteous man sheds blood in Hell. As he breaks, so shall it break.
Dean: Why break the seal anyway?
Castiel: You think of the seals as locks on a door.Castiel: Lucifer walks free.
Dean: Okay, last one opens, and?
And it is written that the first demon shall be the last seal.
Lilith’s blood forms a spiral circle in the middle of the chapel at St. Mary's Convent and the gates of Lucifer’s prison open. 4.22 Lucifer Rising The doors to the chapel close and Sam and Dean are unable to escape. However before Lucifer appears, Sam and Dean find themselves transported to an airplane flying over the area. Joshua reveals to Dean and Sam that it was in fact God who saved them from Lucifer.5.16 Dark Side Of The Moon
Sam and Dean try to kill Lucifer with the Colt but fail, and are desperate for a solution. 5.10 Abandon All Hope In a hidden message recorded on a Casa Erotica DVD, Gabriel reveals posthumously how Sam and Dean can trap Lucifer back in his cage. The keys to the cage are the rings worn by the Four Horsemen of the Apocalypse.5.19 Hammer Of The Gods
Crowley reveals he is aware of Sam and Dean's plan to get the Horsemen's Rings to trap Lucifer back in his cage thanks to his Magical Coin. He also tells them he wants to help them do it. 5.20 The Devil You Know
Sam and Dean get the final two rings after Castiel cuts off Pestilence's finger using Ruby's knife and Death gives his ring to Dean in exchange for his promise to do whatever it took to free him from Lucifer's control, ensuring that Sam would jump into the cage as soon as he consented to possession by Lucifer. Death also instructs Dean on how the rings work to open the cage, although the explanation is not shown in this episode. Dean is later shown working with all four rings on a table. They join together to form a triangular shape with War's ring in the center and the other three rings surrounding it. When Bobby asks him about Death teaching him how the rings work, Dean responds "Yeah, it's nuts."5.21 Two Minutes To Midnight