Dean swaps universes with Dark!Dean? - Found!
I found this fic awhile ago on AO3, and failed to bookmark it at the time, like an idiot. It's fairly long (over 50k, I'd guess). Anyone know what it's called? I'm not having any luck searching for it without more specific tags.
It's a darkfic (gen/non-slash, I think) set around season 4, where Dean wakes up in a parallel world sharing a motel room with an Alt-Sam, who's brutalized and afraid of him. In this world, Alt-Dean came back from hell "wrong" and began beating/abusing Sam, and now the two Deans have swapped universes. Sound familiar? Hoping someone will know what I'm talking about!