"The Soldiers of Halla" by D. J. MacHale (Pendragon, Book 10)

Nine bestselling books. Nine treacherous territories. They have all been leading to this. Every victory. Every loss. The thrills and disappointments; the hope and despair. Every moment of Bobby Pendragon’s heart-pounding journey through time and space has brought him to this epic showdown. He and his fellow Travelers must join forces for one last desperate battle against Saint Dane. Every question is answered. Every truth is revealed. And at stake is not only the tenth and final territory but all that ever was or will be. Everywhere. This is the war for Halla, and the final battle has begun.