Extremist groups are interesting to study. When a society has problems and huge disagreements, there tends to be a rise of extremism which the established power structure tries to suppress, creating the very demons they try to exorcise. These groups can be very complicated and difficult to define. The story should definitely have a few extremist groups that the brothers meet and interesting characters within these organizations they interact with.
For example, since we have crazy cults that worship murderous gods, we could also have groups that champion freedom of religion. Perhaps the aristorcracies are attempting to ban religion, as these cults are getting more powerful in the wake of famine and social problems increasing, but this is opposed by other groups who view religion as a positive.