The mythology of my world is an interpretation of Glorantha. Pretty much near eastern bronze age. So there are Gods abound, with their respective cults. Most/all cults have rites and mysteries for those deeply devoted. None have came up in play so they are secrets even for me.
A set of secrets/mysteries that I’ve started working on are the Nysalorian secrets. According to Gloranthan lore when Nysalor was born/created Time stopped to allow the birth. So yeah Nysalor is a big deal. The Nysalorian secrets I’m divining are about the nature of Godhood. I don’t know how deep I want to go with them, I could make them the very blueprints of Creation. Perhaps I should tie them to my meta-loredump mystic society/cult Followers of the Blind Idiot God.