Supernatural (Season 10)

 In the tenth season, after being resurrected by the Mark of Cain, Dean is now a demon, working alongside Crowley. Meanwhile, Sam continues to search for Dean. After Dean refuses to follow Crowley's order and embarrasses him in front of his demon followers, Crowley gives Sam his brother's location. For giving up Dean, Sam gives Crowley the First Blade. Later, Sam, with the help of Castiel, cures Dean by using sanctified human blood and returns him to his human state. After being cured, Dean is reminded by Castiel that the Mark of Cain is still a problem. Dean and Sam help Castiel track down Claire, Castiel's vessel Jimmy's, daughter. Dean ends up slaughtering several men who are keeping her captive and loses control of the Mark, a nightmare he previously had.

Meanwhile, a mysterious new witch comes into play, being revealed to be Crowley's mother Rowena. Also, the angel Hannah goes back to Heaven so that her vessel can have her life back. A large focus of the season is Dean's quest to overcome the Mark of Cain and have it removed if possible. After the Mark causes him to break Charlie's arm, Sam suggests that Dean has the ability to at least partially control the Mark, but only if he can control himself. New hope for ridding Dean of the Mark comes when Charlie unearths the Book of the Damned. Sam needs help in reading and using this powerful tome, and turns to Rowena for help, keeping the entire thing a secret from Dean. He requests help from Charlie, too, who decodes the text but is murdered by the Steins, a family that has secretly controlled much of history. After massacring the Steins, nearly killing Castiel and getting another hunter killed, Dean starts despairing of being free of the Mark, causing him to turn to Death for help. Death proposes putting Dean in isolation away the Earth, but insists Dean must kill Sam, who otherwise would work to bring Dean back. Sam and Dean both agree that it is for the good of the world. At the last moment however, Dean seemingly kills Death instead to save Sam. Oblivious to the dangers, Rowena successfully casts the spell to remove the Mark and unleashes the Darkness, a primordial evil that had been kept away by the Mark. Rowena also places Castiel under a spell so he attacks Crowley as she escapes with the Book of the Damned.