Lauren Salzman, who pleaded guilty in federal court to charges of racketeering and racketeering conspiracy in March, told jurors that for many years Keith Raniere, 58, was the most important person in her life.
"He was my mentor. My teacher," Salzman said. "We had a romantic relationship. A physical and sexual relationship."
Salzmann has admitted being a member of a secret society within Nxivm known as DOS, in which dozens of women were considered "masters" or "slaves."
Raniere, who has pleaded not guilty to all charges, appeared in court wearing a blue sweater, flanked by his attorneys. He watched as prosecutors questioned the witness.
Salzman did not appear to make eye contact with Raniere while she testified.
She said she met Raniere when she was 21, in 1995. Raniere then became a presence in her life and they had sex for the first time in April 2001, Salzman said.
But that didn't stop Raniere from allegedly having relationships with other women in the group.
"For me, it was a monogamous relationship," Salzman said, adding that she wasn't allowed to see anyone else.
Raniere promised to have children with her as a way to make her stay, but that didn't happen, Salzman said.
She also told jurors that Raniere took a nude photo of her and gave her the target weight of 100 lbs. after getting her to strip down to her underwear and assessing her body.
Nxivm operated like a multilevel marketing scheme, pushing members to take classes that cost as much as $5,000, according to court records.
It encouraged members to take more classes to move up the ranks and to recruit other members to help create more revenue, which left some members in debt to Nxivm itself, according to an affidavit by the FBI.
But while that was going on, DOS meetings called for Salzman and the other women to take nude photos of themselves and send them to Raniere when he couldn't attend a meeting, Salzman said. And when he could attend the meetings, Salzman said, she and others would strip naked and sit on the floor in front of him while he held court, fully clothed, speaking about the group's projects, such as a book or a dungeon they were working on.
Raniere was revered in the group so much that mythology about his ability to control the weather or disable electronics in his presence became legend in the group.
"He would say that his technology was acting up and that he had these types of problems," Salzman testified. "That it was something special about him. That it was his energy. His impact on the world."
Salzman is expected to continue her testimony Monday.