Keys to the Demon Prison (Fablehaven #5) by Brandon Mull
Since ancient times, the great demon prison Zzyzx has protected the world from the most dangerous servants of darkness, including Gorgrog, the Demon King. After centuries of plotting, the Sphinx is on the verge of recovering the five artifacts necessary to open the legendary prison. Facing the potential of a world-ending calamity, all friends of light must unite in a final effort to thwart the Sphinx s designs and find a safe home for the five artifacts. To this end, Kendra, Seth, and the Knights of the Dawn will venture far beyond the walls of Fablehaven to strange and exotic magical preserves across the globe, where the end of every quest becomes the beginning of another. In this explosive series finale, allegiances will be confirmed and secrets revealed as the forces of light and darkness collide in a desperate struggle to control the keys to the demon prison.