Jackelian, book 3: The Rise of the Iron Moon
Stephen Hunt2009

Born into captivity as a product of the Royal Breeding House, friendless orphan Purity Drake suddenly finds herself on the run with a foreign vagrant after accidentally killing one of her guards. Her strange rescuer claims he is on the run himself from terrible forces who mean to enslave the Kingdom of Jackals as they conquered his own nation. Purity doubts his story, until reports begin to filter through from Jackals' neighbours of the terrible Army of Shadows, marching across the continent and sweeping all before them. But there's more to Purity than meets the eye. As Jackals girds itself for war against an army of near-unkillable beasts serving an ancient evil with a terrible secret, it soon becomes clear that their only hope is a strange little royalist girl and the last, desperate plan of an escaped slave.