Former NXIVM member says she was invited into secret sorority then branded

Sarah Edmondson said she attended NXIVM seminars for over a decade when she willingly participated in strange initiation ritual for a sorority that she says was tied to NXIVM.
5:05 | 12/15/17

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Transcript for Former NXIVM member says she was invited into secret sorority then branded
We begin with that investigation into a mysterious group. One former member is speaking out with very disturbing claims and "20/20"'s Elizabeth vargas is following the story. I spoke with a woman with the group nxivm for more than a decade and said she was invited to join a secret sorority allegedly tied to nxivm where she and other women were branded as part of an initial race ritual. We are told the rendezvous was a secret. Five women summoned to a house outside Albany, New York, one night this past March. Sarah Edmondson said she was one of them willingly participating in a strange initial situation ritual led by a woman who told everyone to take off their clothes and put on a blindfold. Led us in blindfolded and sat us in a semi circle butt naked on a sheepskin rug. Reporter: It was earlier this year she said she was approached by a mysterious opportunity to join a secret sorority. Sarah told authorities she thought they were going to get tattoos, instead she says she found out they were all going to be branded. It sounds like a horror movie. It was a horror movie. It was the most inhumane horrific way to treat anybody. What was that like? It was worse than childbirth. It was worse than childbirth. Yep. It was -- imagine a hot laser dragged across your flesh for 30 minutes without anesthetic. Reporter: How in the world did this happen? Sarah lives in Vancouver where she's a wife and mother with a successful film and television career. But her life changed after she crossed paths with the secretive self-help group called nxivm and its charismatic leader Keith raniere. I'm an interesting person, I'm a controversial person but most importantly, I'm an unconventional person. Reporter: According to promotion promotion materials even as a child he was destined for greatness speaking at full sentences at age 1, riding at age 2. Judo champ and concert level pianist at age 13. I was told that he was one of the smartest men in the world and just an incredible man. Reporter: At the heart of it is intensives which former members say are like group therapy. I left my five-day initial training as if a veil had been lifted and could see things more clearly. Reporter: Raniere is fiercely protective of his teachings requiring participates to sign confidentiality agreements. If you didn't sign it you couldn't take it. Reporter: After more than a decade with nxivm she and four other women submitted one by one to being branded by this woman she taste Dr. Danielle Roberts. And the woman on the table screamed out in pain, twisted and turned and yelled and the woman I was with holding her legs down looked at each other and we wept. Why didn't you let go and run out of there even naked? It didn't seem like a choice at the time. Can you just stand up and point on your body through your clothes where the brand is. So, it's right here under my -- Yeah, right there. Right under my underwear line. Reporter: She gave us photos. In her complaint to the health department she says she was told the brand was a Latin symbol but says she realized that it really included the letters "K" and "R," the initials of Keith raniere. I lost it when I figured that out. You're stamped as property of somebody. I'm not owned by Keith. In a letter posted on the group's website raniere says there is no merit to the allegations that we are abusing, coercing or harming individuals. Raniere says that that secret sorority is not part of nxivm and I am not associated with that group. An attorney for Dr. Danielle Roberts, the alleged brander tells ABC news the allegations are unfounded. We'll have a full investigation obviously tonight. What more can you tell us about this group, nxivm? It's been around for more than a decade. The members get very deeply involved. Many have given up career, moved to Albany, New York, where the group is founded and we'll have a lot more and Catherine oxenberg, you may remember her from "Dynasty" and talks about her daughter India part of nxivm, according to Catherine she told her she too was branded and a lot more on Catherine oxenberg's efforts to get her daughter out of the group. More tonight on "20/20." And on "Nightline." Who says I'm an interesting person? Somebody who has an I.Q. Of 240 and is speaking in full sentences at the age of 1. Just like you, George. Right, thank you, Elizabeth.

This transcript has been automatically generated and may not be 100% accurate.

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