S1, E13: Whilst out on a routine hunting trip, Arthur and his men encounter a huge and terrifying monster. It has the body of a lion and the head of a serpent, with a huge pair of dripping fangs. Gaius explains that what they encountered was a Questing Beast, said to have been conjured from the nightmares of a long-dead King. It is a creature of great power from the Old Religion, whose bite is always fatal. Uther refuses to acknowledge such folly and sends the Knights out to kill it. Arthur and his men creep through the forest and find the tracks of the beast. As they cautiously enter its lair the beast jumps out of the darkness at Arthur. Merlin uses magic to send a sword flying at the creature, and it vaporises before his eyes. The beast is dead, but Arthur lies unconscious on the floor, blood dripping from his shoulder – he has been bitten. Arthur is rushed to Gaius – the physician takes one look at the ailing prince says there is nothing he can do, the teeth of the beast have done their evil work. Arthur will soon die. Merlin blames himself for failing in his destiny to protect the Prince. He tries to use magic to save Arthur but it doesn’t work. Gaius tells him that the bite of the Questing Beast is a death sentence which no magic can overturn. The castle goes into mourning, but Merlin refuses to give up, he seeks the help of the Great Dragon who tells him that all is not lost. The Dragon tells him he must go to the Isle of the Blessed and ask the old religion to save Arthur – it alone has the power. Merlin goes back to Gaius with the clues the dragon has given him. Gaius fills in the gaps – the ancient magic which has the power to mirror life can reverse death itself, but there will be a price to pay. Merlin journeys to the Isle of the Blessed where he finds Nimueh waiting for him. Nimueh tells Merlin that she can save Arthur’s life but he must know that a price will have to be paid. To save a life, a life must be taken. A desperate Merlin says that he will take that chance, Arthur’s life is worth anything. If he has to give up his own, then so be it. Nimueh shows him a gold chalice – it is the Cup of Life, blessed by centuries of powerful sorcery so that it contains the very secret of life itself. She tells him that if Arthur drinks water from the cup, he will recover. She gives Merlin some water to take back to Camelot with him. Merlin feeds Arthur the water from the chalice - he slowly shows signs of recovery. Gaius declares he will live. The castle rejoices that their Prince is recovered – and Merlin awaits his fate - but when his mother, Hunith, arrives and is gravely ill, Merlin is distraught. His mother is ill because of the bargain he struck. Merlin already knows what he must do. He tells Gaius if the balance of the world needs a life, it must take his. He will return to the island and give his life in Hunith’s place. An anguished Gaius tries to dissuade him – Merlin’s talents, his destiny, are too precious to be sacrificed but Merlin refuses to listen, he has no choice – he cannot let his mother die. Returning home to collect his things, Merlin finds a letter. It tells him that Gaius has gone to the Isle of the Blessed in his place. Gaius lovingly acknowledges that Merlin’s life is worth more than his and it is his that should be given instead. Merlin thunders after him - will he be able to catch him before it's too late?