Modesitt, L. E., Jr., 1943- Corean chronicles #2

A thousand years after a magical disaster ended the unified rule of the Duarch, Corus is a world of contending nations, struggling humans, strange animals, and elusive supernatural creatures. Much has been lost, but Corus is still a land of magical power, and of a few people who are talented enough to use it, such as Alucius, a nightsheep herder of the Iron Valleys. When the nomads of the central plains attack with the help of sorcerous creatures of legend, Alucius becomes the Militia's youngest captain and best hope for defeating them. The challenges, both military and magical, are severe. Yet a greater one lies ahead, for there are other enemies who would not just conquer a nation, but enslave a world