High in the mountains, a great horde gathers; Morgana prepares for war with a vengeful Mordred at her side. Merlin finds himself facing the future he's fought so long to avoid.

Cast: Colin Morgan, Peter Guinness, Katie McGrath, Alexander Vlahos, John Hurt, Bradley James, Tom Hopper, Richard Wilson, Eoin Macken, Erin Richards, Rupert Young, Angel Coulby, John Lynch