Iron and Leaves
In the Setting
The High Druid is the leader and inspiration for all druids. They are a large, diffuse cult. The center of their power is the Wild Wood, but they are spread throughout the wild places of the Empire and beyond. Perhaps ironically, the High Druid is revered by her followers and is viewed as many as a nearly divine being, the focus of the Wild in the world. Her presence strengthens Nature and weakens the yokes of the Archmage.
In the Rules
According to the rules, the Druids are a Theistic Cult worshiping the Wild. No druid has theistic magic unless they’re affiliated with the High Druid (worshiping nature isn’t enough). If she cuts a druid out, they lose Theism. There are several theories for why that is the case.
I have not yet worked out all the details of the cult, such as lists of available Miracles and Blessings. I expect Shapechaning to be an option.