While out hunting, Arthur and his men encounter a terrifying monster with the body of a lion, the head of a serpent and a pair of dripping fangs. Gaius explains that this was a questing beast, created from the nightmares of a long-dead king. It is a creature of great strength from the old religion, whose bite is always fatal, but Uther refuses to believe such nonsense and sends the knights out to kill it.
Creeping through the forest, Arthur and the knights follow the beast’s trail to a cave. But as they enter its lair, the beast jumps out of shadows at Arthur. As the beast hurls Arthur in a rage, Merlin magically sends a sword flying, killing the beast. But then he notices that Arthur is lying unconscious on the floor with blood dripping from his shoulder after being bitten, and Merlin fears he may not survive…
Merlin seeks the help of the Great Dragon, who tells him he must go to the Isle of the Blessed and ask the old religion to save Arthur – it alone has the power. Armed with the clues the dragon has given him, Merlin visits Gaius, who reveals that the ancient magic that has the power to mirror life can reverse death itself. But there is a price to pay: in order to save a life, a life must be taken.
Will Merlin sacrifice himself to ensure that Prince Arthur lives?