The FBI recently raided the private sex den of a secretive upstate cult leader who allegedly kept women as “slaves” and branded them with his initials, according to a new report.

Days after Nxivm leader Keith Raniere was arrested for sex trafficking and forced labor last week, the feds scoured a Halfmoon townhouse known as “The Library,” where Raniere is accused of abusing at least one of his victims — a 30-something Brooklyn actress, the Albany Times Union reports.

The property, a short walk from Raniere’s residence, has a hot tub and a large bed on the second floor, according to court papers.

The feds say the unnamed actress was duped in 2016 into becoming one of Raniere’s “slaves” by another female thespian — identified by the Times Union as “Smallville” star Allison Mack, a longtime Nxivm devotee.

After collecting incriminating collateral, Mack ordered the 5-foot-5 actress to stay celibate for six months, and then sent her to have sex with Raniere — whose followers refer to him as “The Vanguard,” federal prosecutors allege.

After that, he allegedly started making her travel regularly from Brooklyn to “The Library” for sex. When she told him the frequent trips were becoming too expensive, he pulled $10,000 from a drawer in the townhouse and handed her $1,000, the criminal complaint charges.

When the actress announced she was defecting from the cult last year, he asked for the money back, according to prosecutors.

Raniere, who was busted while hiding out at a luxury villa in Mexico with Mack and other female followers, is being held at a federal prison Oklahoma City and is scheduled to be brought to Brooklyn for the rest of his case.