Ars Magica RPG: Source: The Mysteries Revised

Secrecy is Power

There are Mystery Cults beyond the four Houses within the Order of Hermes. They keep their secrets from other magi, revealing their knowledge one piece at a time to those who undergo grueling initiations. This book reveals all, from alchemists seeking immortality to theurgists trying to become gods. Now your magus can also walk the path of The Mysteries.