The House War, book 1: The hidden city
Michelle West2007

Orphaned and left to fend for herself in the slums of Averalaan, Jewel Markess- Jay to her friends-meets an unlikely savior in Rath, a man who prowls the ruins of the undercity. Nursing Jay back to health is an unusual act for a man who renounced his own family long ago, and the situation becomes stranger still when Jay begins to form a den of other rescued children in Rath's home. But worse perils lurk beneath the slums: the demons that once nearly destroyed the Essalieyan Empire are stirring again, and soon Rath and Jay will find themselves targets of these unstoppable beings.