Image Source: Getty / Amy Luke [1]

HBO's latest documentary, The Vow [2], pulls back the curtain on the horrifying world of NXIVM and its founder Keith Raniere [3], who was accused of running an abusive sex cult through the Albany-based "self-help" organisation. The nine-episode series [4] reveals many of the cults inner workings, which included a subgroup named DOS [5] (Dominus Obsequious Sororium) that was known for branding its members.

The secret women-only inner circle was often described as a "sorority" by its members, which famously included Smallville actress Allison Mack [6], who was Raniere's go-to for recruiting new members. One of the ways the women pledged their loyalty to the group was by getting Rainiere and Mack's initials branded with a cauterizing pen in a ritual ceremony. In a previous interview with ABC's Nightline, former NXIVM member Sarah Edmondson said the branding was Mack's idea [7] and that they were initially told the design was a Latin symbol, though she later realised that wasn't the case. "The four elements were Keith's initials from one side and 'AM' for Allison Mack from dead on," Edmondson explained. "But I didn't see that at the time."

"Imagine somebody taking a lit match to the most sensitive area of your body and dragging it across your flesh slowly," Edmondson later told Inside Edition, adding that the branding lasted between 20 to 30 minutes. "It was horrific. The whole point was to experience pain and to overcome the pain to be really strong, so they cut our flesh open without anesthetic."

In addition to the branding, other means of proving ones sisterhood involved sleeping with Raniere, severe dieting, and giving up a form of "collateral," such as a major secret confession or explicit photos, to ensure that members wouldn't reveal any of the sorority's secrets.