Camelot is about to enter a new golden age, but the dark seeds of destruction are being sown. In the frozen wastelands men have been disappearing without a trace. King Arthur and his knights embark on a dangerous mission to look for answers but once there, Merlin becomes locked in a battle unlike any he has ever seen before.
Some ancient prophecies of Mordred weigh on Merlin's mind as he is unable to sway Arthur from his chosen path. The fortress of Ismere is coming ever nearer and Merlin's fear intensifies. It looks like this time, not even Merlin can stop Arthur from walking right into the lion's den.
After a stranger gives Arthur the power to summon the dead, he ends up being torn between head and heart. He decides to seize the chance to speak to the person he misses most his father. The spirit world is a dark and dangerous place though and little does Arthur realise that his decision comes at a shocking price.
Darkness manages to steal it's way into the heart of Camelot, as Morgana and her puppet Queen come up with a sinister plan to murder the King. After stablehand Tyr Seward becomes caught in the crossfire, he threatens to ruin everything.
After Merlin agrees to help a young Druid boy, Daegal, he is forced to leave Camelot on a dangerous mission. With the citadel being distracted by the visit of the fearsome Sarrum of Amata, it looks like that no one will notice his absence.
Merlin has to move fast before Morgana’s twisted control over Gwen manages to get any further out of hand, but only the most powerful magic can save the Queen. As the High Priestess tightens her grip on Camelot, she is determined not to give up her puppet without a fight.
Morgana heads a shocking manhunt in pursuit her old foe Alator of the Catha, so that he can tell her exactly where to find Emrys. Merlin’s life is in severe danger, so after the mysterious Finna offers to help him, is he right to trust her?.
Mordred is torn between loyalty and love and is clueless to the fact that the path he chooses will shape Camelot's destiny. With the drums of war beating ever louder, a desperate Merlin feels trapped by the cruel circle of fate.
High up in the hills, a great horde gathers as Morgana gets ready for war with a vengeful Mordred. Merlin begins to feel the weight of his destiny like never before as the ancient prophecies play out with shocking accuracy.