Greetings!
It is always interesting to read about various Mystery Cults and different religious cults, usually in an ancient or medieval European context. However, have you considered boning up on Asian flavoured cults for using in your campaigns?
For example, the "Red Turban" cult. A social and religious cult that arose in northern China during the reign of the Yuan Dynasty. More than a few such Chinese cults were very powerful, having many warlords, fortresses, and even armies following under their banners. There were strong bandit groups, river-going pirate ships, secret, mountain strongholds, and more.
The Asian-flavoured historical cults are distinctly different from the European traditions--while also often being religious, and initially secretive, the Asian cults often become far more organized and powerful, and even pose ongoing threats to the survival of empires and kingdoms alike.
Semper Fidelis,
SHARK