I'm looking for a specific AU Sam/Dean fic that I read years ago and I don't think it was on this site.
To save their lives because they were constantly being attacked, Sam accepts a deal to go darkside and joins a brotherhood based in California. He ends up being second-in-command under some sort of vampire, but not like the type Sam and Dean hunted before. While Sam is supposedly evil he is not rampaging around killing everyone.
It's been several years since Sam has made this deal, and Dean has been in downward spiral. He has beeen drinking to much and not being that careful in his hunts. Just as he falls into a drunken stupor outside the Impala Sam shows up. Sam has been watching Dean's decline and decides to bring him to Sam's apartment as his "guest". Sam has a slave who keeps Dean company while Sam is out doing his work. The teenage boy grew up in the 80s and has stopped aging, as part of his agreement to join the brotherhood.
Sam and Dean start a sexual relationship and sometimes the teenage boy joins in.
Dean spends most of his time working on the Impala and trying to find a way to save his brother. He discovers the boy has the ability to send people back in time for a very small periods and people can change the past. He is terrified that the higher ups will find out. He warns Dean not to try to use this power because they would probably both be killed in a bad way.
One night Dean, Sam and the crew go out to dinner and Dean finds himself enjoying himself with these people. They are not the monsters he thought they were. On the way out, they are attacked by hunters, including Jo and Bobby. The teenage boy is shot and they are not sure if he will survive. Sam finds Jo, tortures her and gives her the chance to live if she joins them. Then Sam returns Jo to Ellen and Ash at the Roadhouse, where they have to make the choice to either kill all the other hunters at the Roadhouse because they will kill Jo or help the hunters kill Jo.
When they get back to the complex, Sam's boss is there. They know what Dean and the teenage boy have been doing about trying to change time. Sam's boss uses his powers and takes Dean to a future if Sam didn't make the deal and the world is a disaster. It turns out Dean made a deal with when Sam died to try to get him back and he and in turn the people around him are much cruler. Sam's boss doesn't want this type of a world, he's living well in their world and so are many others. He wants Dean to accept Sam's decision but to stay with Sam because Sam needs Dean to stay sane.