Sky Realm (Hardcover)

The thrilling conclusion to Kevin Anderson and Rebecca Moesta's Crystal Door's trilogy finds cousins Gwen and Vic and their three friends forging the magical Ring of Might and defending all of the worlds beyond the crystal doors in their final battle with the dark sage, Azric.
When their friend Sharif is called back home by his father, the Sultan, Gwen, Vic and their friends travel to the flying city of Irrakesh. When they return, they find that the Sultan is dying, with Sharif the next in line to be Sultan. Soon he will have to choose between duty and destiny, but before he can do so, Irrakesh is attacked and the city is taken hostage by dark wizards evil army.

In their previous battles, they faced Azric by land and sea, now they take to the skies for the climactic showdown with the evil wizard and all of his dark forces.