12x23 All Along the Watchtower

With the birth of Lucifer's son approaching, the Winchesters and Crowley, who survived Lucifer's attempt on his life, search for biblical omens pointing towards the Nephilim's birth. Lucifer kills Rowena so he cannot be sent back to the Cage. At the same time, while taking care of Kelly, Castiel discovers that the baby's power has opened a rift into an alternate post-apocalyptic reality resulting from the Winchesters having never been born. Upon his return, Castiel hides his discovery from Kelly. The Winchesters are eventually able to locate Castiel and Kelly and while Mary tends to Kelly, Castiel takes Sam and Dean through the rift where they are greeted by an alternate Bobby Singer. Bobby is able to supply the Winchesters with a machine gun capable of killing angels to use against Lucifer while Crowley offers a way to seal Lucifer in the alternate reality. As Dean distracts Lucifer, Sam and Crowley perform a spell to seal the rift, a spell Crowley sacrifices his own life to complete. However, Lucifer is able to follow them back through and kills Castiel before the rift can close. Mary is able to send Lucifer back to the alternate reality, but he drags her there with him, leaving both trapped. While Dean mourns Castiel's death, Sam discovers Lucifer's son, born and having the appearance of a teenager.