Pretty obvious their big secret is just a Babylonian people-herding, slave driving technology. A group of Masons with a blood-sworn oath to secrecy? -- quite useful to the man-king whether or not they even know the "big secret." And since we Suitors know all about the methodology behind the enslavement, and used it to escape the pen, where's the secret?
The only big secret left is ... you thinking they have a big secret.