I'm looking for a fic that went AU during the pilot if I remember correctly. During the first season at least.
The demon comes for them in a motel (with ugly flowers on the wallpaper that are creeping them out, which is an odd detail to remember...). Sam bonds himself to Dean with a ritual that just comes to him out of the blue. It makes him Dean's, means he has to obey his orders. He tries to hide that from Dean for a while, who is not thrilled when he finds out. The bond can somehow differentiate between suggestions and real orders, and also means the demon can't have him, because he already belongs to Dean.
Thanks!