S1·E11 Darren's Key

Merlin dreams that Viper possesses a Key of Truth and defeats Darren, and concludes that Darren's key is in Morgana's and the Warlords' domain. Tone builds a hang-glider for Arthur, Lance, Darren, and Trunk to invade Castle Morgana, but Darren falls into the castle grounds and is captured. Morgana intends to use a machine to unleash a potion which will turn him into a Warlord himself, then, learning how many others must still be nearby looking for him, do the same to them.