Merlin faces his toughest challenge yet when Morgana's blinkered determination threatens not only Arthur's future, but the very balance of the world. With her magic stronger than ever, the sorceress summons the mighty Callieach to tear open the veil between the worlds. Hellish creatures pour forth, killing any who succumb to their touch. With Uther a shadow of his former self, it falls to Arthur and his loyal knights, including Lancelot, to protect the kingdom. But it will take more than just swords to vanquish their enemy and Merlin is shocked to the core when he discovers the only way to restore the equilibrium requires a sacrifice of unimaginable proportions.

The mighty citadel is buzzing in anticipation of the prince's birthday celebrations. But Arthur's old adversary, Odin, promises to ensure this is one party Camelot will never forget. For amongst the assembling guests lurks a deadly assassin, set to strike.