Season synopsis
Contains spoilers for this season and earlier seasons.
Dean wakes as a demon after the Mark revives him, and Crowley keeps him close while Sam searches for his brother. Sam captures Dean and starts the cure with sanctified blood. Castiel arrives and restrains Dean when he breaks free, allowing Sam to finish the treatment; Dean returns to human form, but the Mark remains. Castiel's grace is fading, and Crowley resumes ruling Hell as his mother Rowena begins maneuvering around him and the Winchesters.
The Mark makes Dean more violent and harder to reach. Sam seeks a cure in the Book of the Damned, drawing in the Styne family, who want the book for their own purposes. Charlie helps decode its cipher, but the Stynes kill her after she refuses to surrender it. Dean retaliates by killing members of the family, including its leader, and his worsening behavior deepens the rift with Sam. Rowena uses Crowley's attachment to her to advance her plans and secure her freedom.
Sam and Castiel press Rowena to remove the Mark while Dean seeks Death and learns that it is the lock on the Darkness. Death offers to exile Dean far from Earth if he kills Sam; Sam joins them, and Dean turns Death's scythe on Death rather than his brother. While Dean is away, Rowena sacrifices Oskar and completes the spell, removing the Mark and releasing the Darkness. Sam and Dean try to escape in the Impala, but the spreading black cloud engulfs the car and the surrounding field.