Ominous Odyssey (Overworld Chronicles, #13)

But Elyssa smells something fishy and Justin knows better than to ignore her advice. They discover that the mysterious new leader of the Brightling Empire is after a secret weapon hidden in Voltis, a forbidden place where destructive elemental forces destroy anything that tries to get through. If that's not bad enough, the leaders of the last two Darkling legions seem intent on starting a civil war and taking over the government.

Together with Elyssa, Shelton, and Adam, Justin manages to convince the Mzodi to take them to Voltis on a flying ship. It might be a race to their doom, but if they don't find out the secrets of Voltis and keep the weapon out of the hands of the Brightlings, the entire Darkling nation might once again be enslaved.