Three Angels Walk Into a Bar

Lucifer can't take on Amara by himself. Michael is useless, but there's another archangel that he might turn to - one who is none too happy about his big brother parading around in Castiel's body. Oh, or that whole thing where Cas is being tortured instead of left alone. Even if they can beat Amara, how can the boys rescue Cas when the Devil holds all the cards? S11 AU