The Fire of Ares: Spartan 1 by Michael Ford
Lysander is a slave in ancient Sparta, a Helot, but a chance meeting reveals his noble heritage and he is permitted to begin training as a Spartan warrior. The vestiges of his life as a slave are hard to shake off and he struggles to survive the brutal and nepotistic life of a Spartan-in-training. Worse still, his precious amulet, the Fire of Ares, is stolen from him, and with it goes some of his formidable strength. His mother had made him swear he would guard the amulet with his life, without ever telling him why. Lysander is desperate to find the missing jewel, but when he picks up the trail, it leads him to dark secrets about the few people he felt he could trust, and forces him to make a choice between his Helot friends and his Spartan instincts.