Shadowheart (Shadowmarch Series #4)

Southmarch Castle is about to be caught between two implacable enemies, the anchient, immortal Qar and the insane god-king, the Autarch of Xis. Meanwhile, its two young defenders, Princess Briony and Prince Barrick, are both trapped far away from home and fighting for their lives. And now, something is awakening underneath Southmarch Castle, something powerful and terrible that the world has not seen for thousands of years. Can Barrick and Briony, along with a tiny handful of allies, ordinary and extraordinary, find a way to save their world and prevent the rise of a terrible new age an age of unending darkness?