The Sword and the Flame, Dragon King Trilogy Series #3

Sometimes the greatest evil lies within.
The Dragon King who rules the land of Mensandor is none other than Quentin, whose courage and heroism have slowly transformed him from an orphaned servant into a war hero, respected leader, and a fierce man of faith.
But even the powerful can fall prey to weakness.  The world is turned upside-down when the dark sorcerer Nimrood–long thought dead after a battle with the previous Dragon King–returns with a fearsome plan.  Shattered by the death of a dear and trusted friend, the abduction of his beloved son, and the loss of his enchanted sword, Quentin finds his faith tested like never before.
In The Sword and the Flame, the final volume of Stephen R. Lawhead's captivating Dragon King Trilogy, the fate of the entire world depends on the outcome of this climactic battle between good and evil.