Nemesis

It's hard to defend the Earth from deadly monsters when you suspect you might be one of them. Discovering she's a walking magical weapon is just the beginning of Ada's problems. Her guardian thinks she's turned traitor, and her new boss has put her on goblin-catching duty. With enemies around every corner and a centaur uprising threatening to bring a bloodbath to Earth's doorstep, Ada and Kay are once again thrown into danger -- and a conspiracy that could change the world as they know it. Against an enemy they quite literally can't see, Ada and Kay must face up to the real, deadly potential of magic...