The disgraced Smallville star provided feds with an audio tape of a blood-curdling discussion she had with the sex cult leader about how to brand his "slaves" with his initials, prosecutors said in a memo filed in a federal court in Brooklyn Monday, according to Variety.
On the tape, Raniere could be heard asking Mack, "Do you think the person who's being branded should be completely nude and sort of held to the table like a, sort of almost like a sacrifice?"
He also suggested filming the ritual, which he said could be used as "collateral" to prevent the group member from leaving.
"And the person should ask to be branded," he said on the tape. "Should say, please brand me it would be an honor, or something like that. An honor I want to wear for the rest of my life, I don't know... And they should probably say that before they're held down, so it doesn't seem like they are being coerced."
The tape was repeatedly referenced and played by prosecutors during Raniere's trial, though he denied his involvement.
"Although Mack could have provided even more substantial assistance had she made the decision to cooperate earlier, Mack provided significant, detailed and highly corroborated information which assisted the government in its prosecution," prosectors said.
In addition to the recording, Mack – whose role has been identified as "master" of the group – turned over information about crimes committed by other group members as well as provided emails and other documents. She also told investigators that Raniere would ask for nude pictures from the women she had not only recruited to join the group, but also encouraged to have sex with him.
The FBI arrested the 38-year-old actress in Brooklyn on charges of sex trafficking, sex trafficking conspiracy and forced labor conspiracy in April 2018.
She pled guilty to racketeering charges in April of 2019 and is due to be sentenced on June 30.
NXIVM is a now-defunct sex cult led by Raniere that was cleverly disguised as a multi-level management and leadership training program.
He was convicted last year and sentenced to 120 years in prison.
Mack did not testify at Raniere's trial, and her cooperation was not publicly announced.