Heriot by Margaret Mahy

Hardcover children book A legend is born. Heriot has always known that he was different with his terrifying dreams and psychic powers. Ripped from his family farm, he is forced to serve as the King's magician in the capital city of Diamond. Isolated and lonely, his only friends are the 'mad prince' Dysart and Cayley, a mysterious wild city child. But their friendship is about to be tested to the limit when chilling violence and ambition threatens to crush Diamond and its King. Heriot's psychic abilities hide a deeper power that has the strength to bring Diamond's enemies to their knees. But will he be able to control this power without becoming a monster himself?