The Legends of Camber of Culdi #1: Camber of Culdi by Katherine Kurtz

His dream of justice and amicable relations between the races had turned to ashes in his mind. The medieval 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 a century before. Camber could not even save his own son from the murderous treachery of Imre.

When Camber learned that Cinhil Haldane, a descendant of the previous kings, still lived, he realized that the only hope for the kingdom lay in overthrowing Imre and restoring Cinhil to the throne. But Cinhil was a cloistered monk, hidden under his religious name in one of many monasteries, unaware of his heritage, untrained in politics. Could he be persuaded to leave the only life he knew and take on the leadership of a rebellion? And lacking the Deryni powers, could he hope to overcome the magic of the king?

Grimly, Camber set out to locate Cinhil and spirit him from the cloister into a struggle that seemed doomed from the start.

And behind came the minions of the king—for Imre was already aware of the plot and bent on destroying all involved in it.