Icefire (Icefire Trilogy)

Working in his uncle's butchery in the City of Glass, Isandor knows that he has a strange ability. He can turn animals in to ghosts by cutting out their hearts. He knows it's magic and not allowed and has to stay hidden if he is ever become an Eagle Knight.
Then a stranger arrives in town. He is the sorcerer Tandor and he claims that he can help Isandor become a Knight, in return for a gruesome task: to bring him the heart of the Queen.