King Arthur and the Knights of Justice Season 1 Episode 7 The Warlord Knight

Lance and Breeze are attacked by Warlords on their way back to Camelot from a mission, and Lucan succeeds in forcing Lance off a high cliff. While the Knights of Justice try to get over the shock of his loss, the Warlords learn he is actually alive but with amnesia when he approaches them. They take him to Morgana, who realizes that his tumble into the River of Forgetfulness will let them trick him into thinking he is one of them. They claim he is the Warlord Sir William, twin to Lancelot who just defected from King Arthur, whom she convinces him is evil, Everyone is overjoyed when Lance returns to Camelot, but don't know that his surprisingly distant behavior masks that he intends to deliver them into Morgana's hands.