Supernatural Season 5 Episode 12 :

After leaving Dean, Sam Ruby takes a final run to kill Lilith. They abduct Lilith’s personal chef, disguises himself as a nurse, swallow theft newborns for Lilith to her question. Meanwhile, Dean is kidnapped by the angels and in a room without exit, where Zacharias explains that the angels do not try the last seal is broken the deadlock broken up actually want to launch in the Apocalypse, because they clean the earth and bring paradise . So Dean’s task is not to stop the apocalypse to begin, but once it ends. Castiel Dean is furious at him and Castiel deceive reluctantly that the death of Lilith is the last seal to break and send to the Dean of St. Mary’s Abbey, where Sam will be slain only by using his powers to Lilith.

Ruby sees Dean approaches and closes the door so telepathically Dean can not stop Sam. After Sam Lilith slain, their blood is a kind of portal on the ground, while Ruby is Lucifer that work all the time, and the only demon Lilith who knew their true loyalty. She manipulates Sam into killing Lilith, because she knew that Lilith was the final seal. Sam is devastated when he realizes what he has done. Suddenly Dean slams the door and puts Ruby in her own knife and killed it for good, as the door to the prison, the “Lucifer begin to open.

Supernatural Season 5 Episode 12