As dawn breaks across the icy tundra, Merlin and Arthur are close to exhaustion. With each tortured step, the fortress of Ismere looms ever nearer, and Merlin's fear intensifies. What game is Mordred playing? And what powerful secrets are Morgana and the druid Ruadan searching for amongst its twisted catacombs? It seems like this time, not even Merlin can stop Arthur from walking right into the lion's den... For in this bleak and transient wilderness, only one thing is certain: the great trial for Albion has finally begun.

Cast: Colin Morgan, Bradley James, Katie McGrath, Angel Coulby, Richard Wilson, John Hurt, Rupert Young, Eoin Macken, Adetomiwa Edun, Tom Hopper, Liam Cunningham, Stephen McCole, Sophie Rundle, Josette Simon, Alexander Vlahos