I Love Luci...fer

        Once 66 seals are broken by Lilith and other demons working for her, Lucifer, the fallen angel is released from his cage in Hell and brought back to Earth to begin the apocalypse. Both Sam and Dean are in St. Mary's Convent in which the cage was built over and they are in the room where the blood of the first demon is shed allowing him to get out of his imprisonment and find his vessel.
       



Before he can truly do any horrendous damage on the world that he despises, he needs to find a vessel to hold him. His one true vessel is Sam Winchester  however, Sam hates the idea of being the vessel of such an evil being that desires to wipe out half the worlds population and burn the earth to a crisp. 
       




So Lucifer has to substitute in a different vessel that is not as suitable to him as is Sam. One of the reasons why Sam is his vessel is because Sam had demon blood surging through his veins as an infant. This helped him be able to hold down the necessary demon blood Lucifer needs in his vessel to have it stay alive. While on Earth, Lucifer, with the help of the Four Horsemen of the Apocalypse, causes chaos and natural disasters. He intends to destroy humanity and to kill Michael.