The Lucifer Problem

Dean and Sam try to make it work, but after Heaven, it becomes clear to Dean that it would be best to separate. Sam decides to let Dean stop the apocalypse, while Sam takes on the Lucifer problem. After all, if Lucifer doesn't have a true vessel to take, he'll be that much easier to defeat. Sam goes off the grid, and not even Castiel can find him. Pairings: Sastiel