This is quite an old one, but hopefully someone will remember it.
Sam becomes King of Hell, not sure how. He isn't evil but he is very dark. Naturally, Dean is with him, and Castiel too - I think because he wouldn't leave Dean with Sam (or possibly to avoid being taken by Heaven, or both). But now he's reliant on Dean for protection from Sam. (Possible Dean gets powers in this due to being Sam's brother and consort).
I do remember Castiel being scared of Sam and having to play it very carefully.
There are three scenes that have stuck in my mind:
1) Sam follows Castiel into a room and Cas wants to leave but thinks Sam won't let him pass - he does though.
2) Sam is in a relationship with Dean, but wants Cas too - a combination of actually wanting Cas for his own sake, and because he's an angel, and because he's a little jealous of how Dean still cares for Cas. I remember Cas drawing a promise from Sam not to hurt him, and having to remind him of it a few times because while Sam isn't rough he isn't gentle either. Dean's there to support Cas through it and make sure Sam reigns it in.
3)The Host show up to negotiate / deal with Sam, and it ends up with Sam, Dean and Cas on one side of the table and I think Michael and Zachariah on the other. They want to take Castiel back, and claim that Sam is holding him against his will. Sam disputes this, and says Cas is free to go anytime, but I think he gets a few barbs in about how he's treated Castiel better than they ever did. Michael and Zachariah tell Cas his place is in Heaven with them, not in Hell with the Winchesters, but Cas says he'd prefer to stay. (I think he says something like he'd rather serve in Hell than in Heaven). Which pisses them off and I think they threaten to take Cas, but Sam tells them Cas has made his choice and if they try to grab him Sam will just turn them and their armies back at the gates. I remember Cas being quite stunned at Sam being so protective of him.
Thanks in advance!