This Thursday marks the 100th episode of Supernatural, airing at 9/8c on The CW. So what do the Winchester boys have in store to "celebrate" this momentous occasion, you ask? Once they hear of the angels' horrific new plan, they'll be plenty busy alongside Castiel attempting to prevent an all-out war on Earth! And a familiar face returns... but whose side will the mystery guest be on? Check out the preview clip below to get ready for the big 100th ep!