The Brink of Darkness by Jeff Giles

Things have changed for seventeen-year-old Zoe ever since the otherworldly events that brought her together with the mysterious bounty hunter she calls X. In order to save Zoe and her family, X has done the unthinkable – he’s given up his freedom and returned to captivity in the Lowlands. X is determined to break the lords’ hold on him once and for all, but being stripped of his power pushes him toward a darkness he’s never experienced and a past he’s never known. The secrets that surface could be the key to reuniting X and Zoe… or they could mean the destruction of everything they have been fighting for.