Long before organised religion took the shape we would recognise today, the ancient Mediterranean was dense with mystery cults: secretive societies built around initiation, symbolic death and rebirth, and knowledge deliberately withheld from outsiders. Eleusis, Mithraism, the cult of Isis they left few written records by design, and that silence is exactly what makes them so useful, and so dangerous, in fiction.
Researching these traditions meant working from fragments: a relief here, an inscription there, a passing reference in a hostile source. Piecing together what these groups actually believed, and what they were willing to protect, became one of the starting points for the conspiracy at the heart of EGREGORI.