Merlin

Season 4, Episode 1

Episode Title: The Darkest Hour (1)

Air Date: 2011-10-01

Overview:

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.