«Shadows and Light» by Cari Z.

A fight to the death against his former master turns into a chance for Rafael, an assassin denied the immortality he lived to achieve, to learn the shocking truth about the fate he was spared. Rafael wanted nothing more than to serve his immortal master, Xian, a High One of Clare, who taught him from an early age how to be an assassin. But after failing the final test, Rafael was turned out into the Lower City, abandoned by the one person he thought he could count on. Years of hatred and thoughts of revenge have fueled Rafaels quest for vengeance, but when the time comes to strike at Xian, he hesitates and is taken prisoner. Rafael expects to die, either at Xians hand or at the hand of Myrtea, his sadistic mistress. Instead Xian heals him, spares him a brutal interrogation and tells him the dark truth driving the ruthlessness of the High Ones-the source of their immortality is spent. Soon the city will devolve into chaos as High Ones battle one another and the rebelling denizens of the Lower City for power and resources. Xian wants to spare Rafael that pain, just as he spared him the pain of becoming a High One a decade earlier. Their only chance for survival is to escape Clare before civil war breaks out. Even if they make it out of the city, though, there's no guarantee that Xian will live through the agonizing process of becoming human again. The only ones they can rely on are each other…if Rafael can bring himself to once again trust his former master, the only man he's ever loved.