The feds want to seize what they say is the sex den of upstate Nxivm cult leader Keith Raniere and a home where his sidekick, “Smallville” actress Allison Mack, housed one of his “slaves,” according to new court documents.

US attorneys filed paperwork Wednesday to take control of the two properties in the town of Halfmoon as part of the federal sex trafficking case against the pair — who allegedly blackmailed women into becoming sex slaves and branded them with Raniere’s initials.

One of the buildings is a two-story townhouse known as “The Library” where Raniere abused several women — including a 30-something Brooklyn actress who Mack recruited to the group by duping her into handing over incriminating collateral, court papers allege.

Mack ordered the unnamed 5-foot-5 actress to stay celibate for six months, and then sent her to Raniere in the middle of the night to “do anything [he] asked her to do.” He stripped and blindfolded her, tied her to a table, and then allowed someone else to perform oral sex on her while he “circled the table making comments,” prosecutors claim.

After that, Raniere allegedly started making her travel regularly from Brooklyn to Half Moon to have sex with him in “The Library.” While she was in town, the actress stayed with Mack in her condo at 127 Grenadier Court — the second property the feds want to seize.

At least two other slaves had sex with Raniere at “The Library,” which has a hot tub and a large bed on the second floor, court documents allege.

Raniere — whose followers call him “Vanguard” — was paranoid about security at the property, insisting the blinds stay drawn and instructing the actress slave to make sure no one saw her enter or exit.

“When there was snow on the ground, Raniere instructed Jane Doe 1 to make sure to clear her footprints,” prosecutors write.

Raniere evidently kept large sums of money in the building. When the actress slave told him the frequent trips upstate were becoming too expensive, he pulled $10,000 from a drawer in the townhouse and handed her $1,000, the court documents charge.

He also allegedly kept boxes of personal notes there — including ones documenting his sexual partners.

Mack and Raniere have both pleaded not guilty to charges of sex trafficking and forced-labor.