It was called "collateral" — nude photos and other embarrassing material that female members of an upstate New York self-improvement group turned over to their "masters" to ensure obedience, silence and sexual fealty to the organization's spiritual leader, Keith Raniere.

Now some former members of the group, NXIVM, are poised to break their vow of silence for the first time by testifying against Raniere, who has been compared to a cult leader. Opening arguments are set for Tuesday at a federal court in Brooklyn.

Among the more sensational allegations: Some women "slaves" in a secret NXIVM sorority were branded with Raniere's initials as part of their initiation. Others were threatened with the release of their collateral if they didn't have unwanted sex with him.