 Lilith is breaking the 66 seals in order to free Lucifer, and Dean must stop her.Castiel continues to appear throughout the season, at one point sending Dean back in time and later tasking him and his brother Sam with stopping witches from breaking another seal.He returns with fellow angel Uriel at the end of the episode "I Know What You Did Last Summer", seeking to kill Anna Milton, a fallen angel with the ability to "hear" the communications between angels.