Fall of Centuria — James Harris

In the world of Palatovia, a demon lord seeks an ancient artifact, the Book of Stars, in the city of Centuria to gain power over the entire planet. Through his dark arts, he now commands all of the demons and other vulgar beasts, including several members of the royal family now existing as demons. Ptolemy, age 10, counts himself in their number—but the demon lord doesn’t yet know the demon-who-was-once-a-boy possesses what he seeks. While General Gathar and his army fight to get their city and freedom back from the demons, the general learns that what he is afraid of most—magic—is what will save humanity as a new world war begins.