Merlin 10: The Moment of Truth

The Black Knight arrives to interrupt Arthur's coming-of-age celebrations and throws down the gauntlet to any knight who will fight him. He kills them all. Gaius discovers that he is Uther's dead brother-in-law, summoned from the grave by the witch Nimueh, and he is seeking revenge for the death of his sister, who died giving birth to Arthur. He intends to fight and kill the young prince. Merlin learns from the Great Dragon that only a special sword, burnished with the Dragon's fiery breath can kill the knight, and that sword is Excalibur. However, the Dragon tells Merlin that only Arthur can successfully wield it and Arthur has been drugged and locked in his room with his father preparing to take on the knight.