Michael Ausiello has new information on Supernatural 100th episode: "“a very important character will sacrifice themselves in a suicide mission,” reveals exec producer Sera Gamble. “We definitely kept in mind that this was [an important] episode. We wanted to deliver something with real scope; it’s about the Archangel Michael. Crises will come to a head for Dean and Castiel. And you’ll see characters you never thought you’d see again.” "
Source: EW.com.