Gwynedd is coming apart: bandits and corrupt lords prey on the countryside while a self-proclaimed messiah leads an army of fanatics bent on exterminating the Deryni from an impregnable fortress. Lord James Drummond, grandnephew of Camber of Culdi, must stop them virtually alone, working against fanatics whose magic can nullify his own Deryni gifts and a heretic leader who performs real miracles. An officially licensed Choose-Your-Own-Adventure-style gamebook set in Katherine Kurtz's world.
Summary by Loreful, from public sources