Werenight — Harry Turtledove

A young, untried Northern baron must somehow defend his vulnerable realm from an invading force strengthened by darkest magic in this magnificent fantasy adventure from a master world builder

The second son of the Baron of Fox Keep, Gerin never wanted title or responsibility, but both were thrust on him when his father and brother were ambushed and slain by enemy Trokmê raiders. Leaving behind the urban bustle of Cassat and his studies in philosophy and magic, Gerin the Fox returned home to accept his place among the other barons of the Northern marches -- a region secluded from the rest of the Empire of Elabon. But now word is spreading that the Trokmê are returning, led by a powerful new wizard whose magic makes them unstoppable. Keeping his lands and people safe may prove beyond the young lord's capabilities. Only one choice remains for the Fox: He must set out into the wild and make his way back to the city -- for only there can he hope to find a new form of sorcery with which to fight the coming of the Werenight, when men will become monsters.
 
Before Hugo Award winner Harry Turtledove became known far and wide as the king of alternate history, he introduced himself to the world with Werenight, carrying readers into a remarkable realm of war and magic. Filling his fantasy adventure with a colorful cast of unforgettable characters, Turtledove has created a fantastic reality as rich, involving, and imaginative as the histories he later so ingeniously altered.