Demonkin (Demonstone Chronicles #4‪)‬

As the world prepares for war, a small group of Alceans plot to free the elves from the grip of the enemy. The goal is to make the elves disappear, but over a thousand elven children are held captive in the Federation, and the elves will not flee while their children are endangered. Complicating matters are the 'births' of a new breed of demonkin whose goal is to kill the heroes of the Mage.