SEX cult actress Allison Mack breaks down in tears as she sings a bizarre love song for leader Keith Raniere on his birthday in this creepy video footage obtained by Sun Online.

The former Smallville star, who earlier this week pleaded guilty to her involvement in the sinister NXIVM group in which women were branded on their groins, can be seen teary-eyed and struggling to keep a hold of her emotions as she performs a version of "I Love You More Than You'll Ever Know" by Donny Hathaway.

6 Mack sobs as she performs a love song for "cult leader" Keith Raniere's birthday Credit: Frank Parlato

6 She broke down in tears singing the love song Credit: Frank Parlato

Raniere, currently in jail facing a slew of federal charges including sex trafficking, can be seen in the audience watching the a cappella performance, which took place around the same time she began recruiting women for DOS - a secretive master-slave sect at the heart of NXIVM.

In the video, Mack is barefoot and wearing a loose white dress as she sings of her love for the cult leader with a group of backing singers.

The actress, currently facing up to 40 years in prison, then breaks down in tears as she sings: "I am only flesh and blood but I can be anything you demand."

Frank Parlato, who first published the video on his blog Frank Report, said the footage was taken around the same time that Mack began her "criminal behaviours".

Earlier this week Mack confessed to being part of DOS, admitting that she kept a "slave" and instructed women in the group "to perform services for me."

Prosecutors allege that women in DOS were kept on starvation diets and groomed to have sex with Raniere.

Former members of DOS have told how they were held down while they were branded with Raniere's and Mack's initials with a cauterizing iron.

They were also forced to hand over damaging collateral such as naked pictures, confessions and financial information - as well as have sex with Raniere.

Parlato told Sun Online: "This performance takes place on Raniere's birthday during the 2016 Vanguard Week, an annual event dedicated to him, and you can see she is deeply moved.

"She's singing these words that are especially meaningful to him - and I think what is particularly intriguing about the video is that it was taken in 2016. By then DOS had begun.

"When she pleaded guilty this past week she admitted that her criminal behaviours began about this time.

"So her singing barefoot in front of Raniere shows the depth of her complete integration into the cult of Keith Raniere."

6 Keith Raniere is accused of being the mastermind behind the operation

6 Women were branded with Mack and Raniere's initials on their pubic areas

Parlato, who first exposed the branding and worked tirelessly to expose the cult, says it's a huge turn around for the star, who has now publicly turned against her once beloved master.

"She has made a 100 percent denunciation of Raniere now but that could be based on practical considerations," he said.

"He is going to go to prison for a very long time - she is facing a shorter or longer time depending on how much help she provides to the prosecution.

"She has signed a 10 page cooperation agreement with the US government to help their prosecution against the remaining defendants - including Raniere.

"I think the turning point for a lot of his supporters was when the government discovered a hard drive at his home containing explicit pictures of a 15-year-old Mexican girl.

"When that evidence came in everyone gave up on Raniere - including Allison. She knew that she didn't want to go down with the ship."

Parlato also revealed how he and actress Catherine Oxenburg - whose daughter India was also part of NXIVM and DOS - repeatedly begged Mack to leave the group, before there were any arrests - but she refused.

"I believe Allison was both a perpetrator and a victim - if she hadn't come into the path of Keith Raniere she would have continued with her acting, had her on TV show maybe even gone into movies.

"Now she's lost everything. But at the end of the day both Catherine Oxenburg and I reached out to her repeatedly and implored her to leave before any arrests were made - or before anybody knew about the branding.

"She ignored all my calls and refused to communicate with me but I sent her messages repeatedly.

"Catherine and I talked to her manager. We had a long two hour conversation with her to try and persuade her to get out.

"If she had left then I don't think she would have been arrested - India Oxenberg left at that time and agreed to testify for the prosecution and she avoided arrest."

Mack, who pleaded guilty to racketeering and racketeering conspiracy charges — including extortion and forced labour - is now facing a maximum 40 year sentence but she's more likely to get around five years, according to Parlato.

Then he believes the-once successful Hollywood actress will try to reinvent her career.

"The maximum sentence is 40 years but she won't get that - she's more likely to get around three to five based on US sentencing guidelines for the crimes she has confessed to and her lack of criminal background," he said.

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"I think her lawyers are going to now try get her sentence down to two years or less and try and get her qualified for home arrest.

"Or she'll try and get a low security prison where she won't be subject to unduly harsh living conditions.

"After that I think she'll try make a comeback with a strong defence that she was brainwashed and that she followed Raniere with the best of intentions."

6 Mack pleaded guilty to racketeering charges in front of a judge earlier this week Credit: Getty