Merlin - Season 1 Episode 2

King Uther Pendragon holds an annual tournament where Prince Arthur Pendragon is to defend his title against other knights of the realm. Sir Valiant comes with a magic secret weapon – a shield from which the painted poisonous serpents can come to life and attack (which Sir Valiant also used to murder its creator). By nursing snake-poisoned knight Ewan, Merlin finds out how this vile knight has been defeating adversaries as easily as Arthur. He informs the prince, but a servant's word is worthless against a knight's, and by the time the prince challenges Valiant, the only 'worthy' witness has died (via more serpentine magic). Arthur, furious to be humiliated and seeming scared, fires Merlin. Receiving enigmatic encouragement from the Great Dragon, Merlin decides against giving up, resorting to magic from the book Gaius earlier gave him.