I can't remember where I read this, in it Angels were like a ruling class and Dean went as some sort of pet/slave to Cas for a reason related to Sam, possibly in his place but maybe not. I remember Sam was sick and was somehow restored to health. Cas was a good guy in this, and he and Dean went about completing some sort of list of tasks, many which seemed impossible at first, for some cosmic purpose or other. I remember there's a scene where Dean and Cas have to interact with other angels, and Dean sees the other human "pets" and that they are being treated far worse than he is.

If anyone were to remember this and let me know where I could find it, I'd be very thankful!