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 guarantee obedience, silence and sexual fealty to the organization’s spiritual leader, Keith Raniere.

Now some former members of the group, called Nxivm (pronounced nexium) are poised to break their vow of silence 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 female “slaves” in a secret Nxivm sorority were, literally, branded with Raniere’s initials as part of an initiation. Others were threatened with the release of their collateral material if they didn’t have sex with him, and only him.

Prosecutors have been tight-lipped about who will testify about the secret so-called sorority group that formed within the group, called DOS, an acronym for a Latin phrase roughly translated as “Lord/master of obedient women”.

There is speculation that former members of Raniere’s inner circle including the TV actor Allison Mack and Lauren Salzman, the daughter of the group’s top executive, Nancy Salzman, could take the witness stand against him.

US district judge Nicholas Garaufis has yet to rule on a request by prosecutors to protect the privacy of some alleged victims, referred to as “Jane Does” in court papers, by only using their first names, nicknames or pseudonyms while they testify, to protect them from “potential harassment” and “undue embarrassment”.

Raniere, 58, has pleaded not guilty to sex-trafficking and other charges. His lawyers have opposed what they call “unusual and dubious” protections that would violate his constitutional right to confront his accusers. They add it would “unfairly signal to the jury that, in the court’s view, the witness is a victim of a sex crime who is in danger”.

The actor Allison Mack leaves Brooklyn federal court in New York after pleading guilty to racketeering charges on 8 April. Photograph: Mark Lennihan/AP

Either way, the case has resulted in a dramatic downfall for Raniere from a time when he was known as “Vanguard” by devotees in the United State and Mexico.

Promotional material for the now-disbanded Nxivm once hailed him as a “scientist, mathematician, philosopher, entrepreneur, educator, inventor and author” who had “devoted his life to developing new tools for human empowerment, expression and ethics”.

Last year, after a New York Times exposé on the group and reports that investigators were interviewing some women who had defected from DOS, Raniere fled to Mexico. He was ultimately found staying with Mack and other women in a luxury villa in Puerto Vallarta and taken into custody on a US warrant.

Mack, best known for her role as a young Superman’s close friend on the series Smallville, also was charged, along with Clare Bronfman, an heir to the Seagram liquor fortune, who bankrolled Nxivm. Salzman and two others also faced charges.

But after prosecutors added child exploitation charges against Raniere earlier this year, based on evidence he had sex with a 15-year-old girl, all of his co-defendants pleaded guilty.

Salzman admitted in her guilty plea they she held a Mexican woman hostage in an upstate home for more than two years under threat of having her deported, “if she did not complete labor requested by myself and others”.

At her plea hearing, Mack tearfully said she had collected “collateral” against women and regretted getting involved with Raniere – a change of heart from when she told the New York Times that, as one of the DOS “masters”, she saw the women’s servitude and willingness to be branded as acts of devotion.

Authorities say the branding was done using a cautery pen without anesthesia by a doctor who is now under investigation by state health officials.

Eight “Jane Does” in that case have refused to answer questions, saying through their lawyers “the branding was a voluntary free expression of personal beliefs”.

In the court papers, defense lawyers have similarly claimed the alleged victims were never abused. The women are instead described as “independent, smart, curious adults” in search of “happiness, fulfillment and meaning”.

But legions of Nxivm defectors and detractors have called the self-help rhetoric a brainwashing device that has wrecked lives.

Among the most vocal opponents has been the TV actor Catherine Oxenberg of Dynasty fame, whose daughter is a former Nxivm member who once revealed to her mother she had been branded and considered it “character building”.

Hearing that, “My heart broke into a millions pieces,” Oxenberg said at a recent forum on the subject. “I knew I had lost her.”