As I understand it, a governor can improve happiness/loyalty with piety or dread. If the province to be governed has a low percentage of your religion’s followers, then dread is used. If the percentage is over 50%, then piety takes over. I believe that a faction ruler’s piety affects whether rebels (the faithful), in a former province who are attacking your enemies, will automatically rejoin your realm. So what does a faction ruler’s dread do?