I was pretty sure I'd saved this one. But it seems I didn't and a quick look at my AO3 history shows that I ight have been logged out of my account when I read it because is not there either.

I think it's a recent one. But it's an alternate take on s05, instead of God getting Sam and Dean out of the church when Lucifer is rising, Dean disappears. Another Sam in an alternate reality made some deal with Lilith to take a Dean from a different world because he had to kill his own Dean, who was a demon by then. Lilith promises that she's not taking Dean from a Sam that loves him, but Sam ends up realizing that the other Sam does lobe his brother. So he ends up sacrificing himself and Dean is returned to his own world. They also end up getting that universe's Castiel to help.

When I first found it was late so I just read quickly over it and I was hoping to sit down and read it fully. I've spend the last hour on AO3 trying to see if I can find it, but no luck. Maybe someone read it and actually bookmarked it.