Season 5

Season 5, which is currently still airing, has the two boys trying to hunt down and kill Lucifer. We learn that Dean is to be a vessel (body in which a spirit resides so they can walk on earth) for Archangel, Michael. The reason for this is because Michael, portrayed as Lucifer's brother in Supernatural, is the only one who can kill Lucifer. Dean must accept the request from Michael in order to become the vessel and Dean refuses, at least through the episodes currently aired. We also learn that Lucifer is walking the earth but is in a weak vessel. He needs Sam to be his vessel to reach his full strength, but also needs Sam to accept the offer. There are currently different signs of the Apocalypse occuring throughout the show, one of which is at least three of the four horsemen of the Apocalypse being seen. The next episode airs March 25.

Sam and Lucifer - http://static.tvfanatic.com/images/gallery/sam-and-lucifer.jpg