Nabari no Ou(2008)

In the shadows of this world, ninjas fight for control of an ancient technique which holds untold strength. This coveted power dwells within apathetic Miharu, a fact the guy really couldn't care less about - until the rival clans bring their battle to him. Now, he must choose a side if he hopes to survive - but when conflict is waged in secret, friend and foe prove difficult to tell apart.