On the Head of a Pin
Supernatural Season 4 Episode 16 (03/19/09)
Someone finds Lucifer's sword and uses it to kill angels. Castiel and Uriel capture Alastair and try to get Dean to use his torturing skills to learn how to stop the murders. Sam is concerned Dean can't do it, but Dean agrees. When Alastair reveals some shocking information, Dean's world is turned upside down.