S1, E6: During a mission to the Centre of the Earth, Gus' powers mysteriously begin to fail while fending off Zarlak's forces. Investigation reveals that the magic-loss is affecting Gus' whole Elfin family. Gus' father reveals that their magic originally came from the Elvenstone, a mystic gem stolen long ago by the evil dragon Wyrmost, who was banished to the Centre of the Earth with Zarlak...