All I remember was that Dean said yes to Michael, and Michael promised that after he's done with Dean, he'll leave Dean not a drooling mess. And while Michael is using Dean as a vessel, he starts fucking Castiel. And I don't remember what exactly happened, but Michael was using Castiel and broke him mentally or something. When Dean regain his body back, he's expose to a really needy Cas that just wants to be fuck constantly(myb?). But Dean's totally fine with it and he might even like Castiel being like that. I have this itch that the fic in on livejournal.