Camber the Heretic (The Legends of Camber of Culdi, Vol. 3) by Kurtz, Katherine - 1981

"Camber of Culdi, most powerful of the magical Deryni race, was worried. Twelve years before, he had 'died' and assumed the form of Bishop Alister Cullen, in order to guide the human king Cinhil and ensure peace between Deryni and humans in Gwynedd. In most ways, he had been successful.
"But now Cinhil was dying, leaving his sickly son Alroy, in whose name a council of regents would govern. And Camber knew those regents to be evil men, greedy for power and filled with hatred for all Deryni. Already the persecutions were beginning, with terror, rapine and genocide soon to follow. Even the Church could offer no sure refuge.
"The only hope of Camber and the Council lay in a discovery by the Healer Rhys to block off all Deryni talents. So blocked, some few Deryni might go underground, undetectable as other than human. But that meant the race of Deryni could be saved only by being destroyed as Deryni! "; 6¾" - 7¾" Tall .