Shine of the Silver Dragon (Dragon Masters #11)

In the eleventh book in this fantasy series, the wizard Maldred is back! And Diego is under his dark spell! Drake learns that Maldred wants to control the Naga -- an enormous dragon that lives deep within the Earth. Two keys are needed to control the Naga. The first key is guarded by a Silver Dragon named Argent. Drake, Bo, and Carlos must find Argent and stop Diego before it's too late. But will Argent's powerful Dragon Master Jean get in their way?