Filth nuns are generally educators and givers of advice. They teach newer members intricate details about the cult, about advancement in the cult, about rituals, and about the cult's magics
Some filth nuns also perform some rituals, typically smaller, less ceremonious versions of the Detrimid Ritual.
It's important that a filth nun be recognizeable, so they tend to wear habits like an actual nun. These are typically black, with more important filth nuns having red ones.