From what I remember, this fic was in the form of Cas (Lucifer)'s letters to Michael. Basically, he kidnapped Dean to Hell because he was Michael's vessel, but over time, he started to fall in love with him. Michael sent a garrison (led by Zachariah, I think..?) to rescue Dean, but it turns out that Cas was taking care of Dean and was angry that Dean didn't have a good childhood. There was a part where  Cas mentioned that the garrison provided the members of Hell nice, fluffy feathers for mattresses and pillows. Anyway, later on, Cas (Lucifer) rises from hell and Jimmy Novak becomes his vessel. Dean renames him Castiel because he says he should get a fake name for a cover. Cas and Dean marries and Michael was invited to their wedding, and Dean sent Michael homemade pie. 

I hope it sounds familiar...