I started reading this fic ages ago and then I lost the link and I can't find it for the life of me.
It was an AU starting from Mommy Dearest where Sam's wall got taken down and along with the memories from the cage he remembered that he is actually a fallen angel named Sammael. It was a Destiel fic, too, and the author had done loads of research on Leviathan and Behemoth and illuminated manuscripts and stuff. I think it was a WIP, too.