Castiel is back on the episode "The Third Man." Check out this clip from the 10/8/10 episode of the show.
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Check out the official promo for The Third Man. On the episode, Sam and Dean call on Castiel for assistance as they investigate a case about several dead police officers.
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Supernatural Season 6 Episode 3 Quotes

Now I lay me down to sleep, I pray to Castiel to get his feathery ass down here.

Dean

Dean and I do share a more profound bound. I wasn't going to mention it.

Castel