Arthur's Bane, Part 1

S5 E1: 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.