The Goblin Mirror — C.J. Cherryh

Things weren't right in the little kingdom of Maggiar, so the princes Bogdan and Tamas set off to seek an answer to the kingdom's troubles in the world over the mountain, a world they knew only from legends. But that glorious place was great no more. The goblins had declared war, and no one--especially not visitors were safe from the raging battles, the darkest sorcery, and the evil that roamed the land....