Camelot faces a new golden age, but even as the city flowers, the dark seeds of destruction are being sown. In the frozen wastelands of the north, men are disappearing without a trace. King Arthur and his knights undertake a dangerous mission into the unknown in search of answers - but once there, Merlin finds himself locked in a battle unlike any he has fought before.

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 (Liam Cunningham) 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.