Draconis Academy 2: Inverse Earth (A Nocturnal Academy Story) - Draconis Academy, #2

The Nocturnal Academy principal has organised another special excursion, this time to the Hollow Earth. He wants to visit the exiled Vrill-ya Darmin Gallis, and perhaps entice the introverted people from their ivory towers. He brings with him five students - Drago, Colin, Olive, Derrick and the fairy Auberia. They are all eager for adventure, but the head master informs them that this is just a routine trip to earn some extra credit.
Little do they know that an old enemy has risen from the dead and has a diabolical plan of revenge planned for both Gallis and the entire Vrill-ya race. This ancient people, responsible for the construction of the Hollow Earth, must now rely on an exile and a party of Surface Dweller supernaturals to help them defeat this evil demon king.