For 20 years, thousands of eager adults signed up for personal and professional development classes through a little-known upstate New York company blandly called Executive Success Programs, or ESP.

Started in 1998, the lessons — beginning at five-day sessions — taught students to change habitual behaviors, reject needless fear and gain full control over their response to any situation. Empowering as that sounds, however, these teachings had a much more insidious goal.

In 2017, it was revealed that the workshops, operating near Albany under an umbrella group called Nxivm, were concealing a reprehensible sex cult. It was later alleged in high-profile criminal prosecutions of several Nxivm members that pretty young women were turned into slaves, made to eat restricted-calorie diets, forced into bed with the company’s founder and, most barbarically, branded on their pelvis with the perv’s initials.

The rise and fall of Nxivm is the subject of a captivating nine-part HBO documentary series, called “The Vow,” which starts Sunday night and exposes how one person’s charisma and calm-voiced coercion can destroy countless lives.

Nxivm was unlike any cult you’ve heard about before. For one, it was not a religion or a commune, but a reputable business, albeit an alleged pyramid scheme. Even now, it is repeatedly referred to by its defectors as “the company,” as though it offered health insurance rather than bunk science and PTSD. Bucking the usual cult cliches, Nxivm’s members were not financially struggling runaways, but millionaires, TV stars and even royalty. Seagram Company heiress Clare Bronfman reportedly bankrolled Nxivm to the tune of $150 million over 15 years.

And they were all in the snare of Keith Raniere, a short, bespectacled, Brooklyn-born man with a Jesus haircut and hypnotist’s voice. He called himself “Vanguard,” and everyone else had to as well. It’s creepy.

Filmmaker and former high-ranking member of ESP, Mark Vicente, whose wealth of archival footage makes the doc extraordinary, said of their strange first meeting, “There was part of me that was like, ‘This is the dude?’ ”

A pianist, judo master and self-styled scientist, Raniere, now 59, also enjoyed kissing everybody on the lips as a greeting.

“If this guy created all these things that are helping so many people, who cares about the kissing-on-the-lips thing?,” Bonnie Piesse, the actress who played young Aunt Beru in “Star Wars: Revenge of the Sith,” said in the doc.

Women were especially transfixed by Raniere’s ideas and often fell in love with him. Early in the organization’s history, he was having sexual relationships with at least 12 female employees at once, according to a spurned lover in the documentary.

At their compound in Albany, the founder would host bizarre late-night volleyball games past 1 a.m., an odd obsession of his, and followers would flock to the gym to compete and ask him questions. As he rambles New Age gobbledygook, we see Nxivm members shower him in adoration in footage captured by Vicente.

These theories that so enamored the crowd — that pain could feel good, or that fear was a choice — would later be used to enforce sexual servitude.

Piesse, who was recruited by and later married Vicente, recorded a conversation between her and Raniere during a walk one evening. She was expressing to him her concerns about her job as a “proctor,” working 23-hour days for no money with little time to eat, leading her to nearly pass out during a Nxivm singing rehearsal.

“If you pass out, you pass out,” Raniere said. “I’ve done that. But when there are higher values, when you’re connecting with people, when you are serving humanity, which is serving your inner self? And that sort of servitude is not slavery.”

The leader said, “What are you scared of? What’s really gonna happen to you? I’m not going to put you in a chamber and torture you.”

Longtime members Piesse, Vicente and Sarah Edmondson unceremoniously left Nxivm in 2017. Their move was risky because the group is known for being highly litigious, breaking and entering into former employees’ homes and seeking revenge. But the departure was unavoidable after they made the disturbing discovery of a faction called DOS.

Nxivm contained many different specific entities. Jness was a subset meant to embolden powerful women, while the SOP (Society of Protectors) was a macho bunch of dudes building the leaders of tomorrow; Exo/eso was exercise- and arts-focused. Thousands of people took classes, and the most dedicated ascended the “stripe path,” a tiered, color-coded system of seniority determined by recruitment and dedication.

But DOS wasn’t mentioned on the group’s website or fliers.

Started by Raniere and “Smallville” actress Allison Mack, Dominus Obsequious Sororium was a horrific secret society of “masters” and “slaves,” in which one woman would recruit another woman and then have total control over everything she did, according to court papers. To join, which is described in audio recordings as a high honor, women needed to continuously provide “collateral” — usually nude photos and damaging information. Sometimes, they signed over the deed to their home.

Once the woman made the sacred vow, she was branded like cattle with a symbol combining Raniere and Mack’s initials. In Edmondson’s case, she was taken blindfolded to Mack’s house for the two-hour ritual, and then forced to watch her friends scream as their flesh was seared before her own torture.

“I was just thinking, ‘How the f–k am I gonna get out?,’ ” Edmondson said in the doc. “And they weren’t doing well. They were squirming, they were crying, they were twitching, they were sweating. And at one point Lauren [Salzman, daughter of Nxivm co-founder Nancy Salzman] pulled me aside and said, ‘You’re green [a level in the company]. You need to show them how to do this.’ ” Edmondson had to be the example, and choose to not feel fear.

‘I was just thinking, “How the f - - k am I gonna get out?”‘

 - Sarah Edmondson

In the doc, viewers learn about “Readiness Drills,” in which a master texts her slave, “?” and then the slave must reply, “RM,” short for “Ready, Master,” within one minute. Before every meal, calories must be counted and approved by the master. And, in one of the sickest assignments, the women were tasked with seducing Raniere and having him take a nude photo to prove the encounter occurred.

All the while, they were forced to continue doing the day-to-day work of Nxivm for little to no money, and the perv-protecting organization kept teaching as if nothing was going on.

The group’s wrongdoings finally came to light in 2017, first by a blog called the Frank Report, run by Nxivm’s former publicist, Frank Parlato, and then in the New York Times, The Post and other publications. Although the efforts of Edmondson, Vicente and Piesse to expose the organization’s wrongdoings are heroic, this is where the series starts to lose steam.

It’s moving to witness actress Catherine Oxenberg work to rescue her brainwashed then 26-year-old daughter India, who was branded and made a slave, from the clutches of Nxivm, but we no longer get as many rare glimpses into the mysterious organization, which would collapse in 2019 after Raniere was found guilty of sex trafficking, forced labor and racketeering. Mack pleaded guilty to racketeering, while Bronfman, who bankrolled the group, pleaded guilty to harboring illegal aliens. Bronfman and Raniere both await sentencing.

Still, the elder Oxenberg gets the most memorable line in the show, when she is on the phone with her mother, Princess Elizabeth of Yugoslavia.

“Can you call Charles and get him in direct contact with the Dalai Lama?” Oxenberg said on the phone. “I want to tell [the Dalai Lama] that my daughter is being held as a sex slave to this organization that he received money from.”

“Who’s Charles?,” asked the producer.

“Prince Charles,” said Oxenberg. “He’s mom’s second cousin.”