Queen Morgana begins a reign of terrible evil over the innocent citizens of Camelot. With Morgause's immortal army at her command, it seems there is no hope for Uther, who must watch his kingdom crumble under the unimaginable cruelty of his own daughter.
Meanwhile, on the outskirts of Camelot, Arthur regains his strength and, with Gwaine, Lancelot, Elyan and Percival at his side, plans a brave counterattack. But Merlin knows that even Arthur cannot vanquish an immortal foe. Can he recover the great sword Excalibur from the Lake of Avalon in time to save Arthur from certain doom?
Starring: Colin Morgan (Merlin), Angel Coulby (Gwen), Bradley James (Arthur Pendragon), Katie McGrath (Morgana), Anthony Head (Uther Pendragon), Richard Wilson (Gaius), John Hurt (Voice of the Great Dragon)
Guest Starring: Emilia Fox (Morgause), Santiago Cabrera (Lancelot), Eoin Macken (Gwaine), Rupert Young (Sir Leon), Adetomiwa Edun (Elyan), Tom Hopper (Percival), Laura Donnelly (Freya)
Written by Julian Jones
Directed by Jeremy Webb