Camber the Heretic (The Legends of Camber of Culdi, Vol. 3)

King Cinhil, who had assured peace between the Deryni and humans in Gwynedd, was dying. And the regents who were set to rule in the place of his sickly son were evil men who could very well undo all that Cinhil had accomplished. Once they ruled, no Deryni would be safe. The only hope lay in a discovery that blocked off all Deryni talents, enabling them to go underground and appear as humans. But that meant that the race of Deryni could be saved only by being destroyed as Deryni...! "Camber of Culdi, most powerful of the magical Deryni race, was worried. Twelve years before, he had 'died' and assumed the form of Bishop Allister Cullen, in order to guide the human king Cinhil and ensure peace between Deryni and humans in Gwynedd. In most ways, he and the Camberian Council he headed had been successful. But now Cinhil was dying, leaving the kingdom to his sickly young 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. Once they ruled, no Deryni would be safe. 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, who had learned - without understanding -- to block off all Deryni talents. So blocked, some few Deryni might go underground, undetectable as other than merely human. But that meant that the race of Deryni could be saved only by being destroyed as Deryni!"-- From publisher's description