Camber of Culdi (The Legends of Camber of Culdi, Volume 1)

Camber was the greatest of the Deryni -- that race of men who were gifted with arcane mental powers that set them above normal humans. In later legends, he was to become a figure of mystery, known as both the patron saint of dark magic and the defender of humanity. But now he sought only retirement on his family estates. But it was not to be. The kingdom of Gwynedd groaned under the tyranny of Imre and his sister and mistress, Ariella. Normal humans were savagely persecuted by the king, whose Deryni ancestors had seized the throne from the rightful human Haldane line. Camber could not even save his own son from the murderous treachery of Imre. Then Camber learned that Cinhil Haldane, a descendant of the previous kings, still lived, and he realized that the only hope for the kingdom lay in overthrowing Imre and setting Cinhil on the throne. Grimly Camber set out to locate Cinhil and spirit him away from the only life he ever knew to lead him into a struggle that seemed doomed from the start.