The Knife's Edge by Matthew Wolf

When legends come to life the world trembles from a single name. Ronin. Once-heroes from a different age, they wield elemental powers… wind, water, fire, stone, forest, sun, moon, flesh, and metal. At the same time, a young man discovers his best friend with a sword in her stomach, and dark wings sprouting from her back. Guards rush onto the scene, accuse him of the act, and he is forced to flee. In a new world without his memories, Gray must find his way amid legends and darkness, as he wrestles with an elemental power inside himself. A power all too similar to the infamous Ronin…