Evil sorceress Morgana summons mighty gatekeeper the Cailleach, who opens a door to another world from which hellish creatures pour forth. As Arthur and his loyal knights take up arms, Merlin discovers the only thing that will stop them is an unimaginable sacrifice. (TV Guide UK)
U.S. air dates are in MM.DD.YY form. U.K. air dates are in DD.MM.YY form.
