Darknesses: The Corean Chronicles Book 2 by L. E. Modesitt Jr.
A magical calamity spread a dark age across the planet, leaving Corea a divided, warring world where kingdoms fight and people live amongst supernatural beings and bizarre animals. A few rare humans have Talent, the ability to use magical powers, and are thus either cursed or blessed.
Alucius is one of the most Talented of all, and is now rising quickly through the ranks of the Iron Valleys Militia. He has to have all his wits about him to survive in a complex political landscape, having spent his childhood on an isolated nightsheep ranch. He must stretch his Talent and military skill to their limits, as he battles to keep himself and his men alive in a a fierce border war, while the greatest evil yet awaits him where he least expects to find it.