Harn is a ridiculously detailed world, very different from Glorantha but of a similar magnitude. However, you can easily get a lot of good stuff out of it which can be used in BFRPG or other fantasy settings.
https://www.lythia.com/hrt/morgath/prac ... rgath.html
What makes Morgathianism attractive as a religion? In order to understand why anyone would willingly place his or her faith in a god who advocates the destruction of the known universe, one must first examine the logic of the argument. At its most basic level, Morgathianism is a religion of anger and blame. Post-Balshan Morgathianism teaches that the universe is the way it is because of order. If someone is dissatisfied with their station in life, they are more likely to be open to a dogma that teaches that it is not their fault. If they are feeling oppressed or powerless, they are more open to a belief system that teaches them that the oppressive social system is inherently flawed.