What are some of the secret religious rites in your world?

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.