Federal prosecutors say the one-of-a-kind Wu-Tang Clan album once owned by convicted former pharmaceutical executive Martin Shkreli has been sold at auction to an anonymous buyer for an unknown price.

"Once Upon A Time In Shaolin" is unique in a few ways: it's never been commercially released, there's just the one copy encased in a silver box, and only a handful of people have ever been allowed to hear it. Shkreli bought the album at auction in 2015 for $2 million. Just a few months later, he became known as "Pharma Bro" when his pharmaceutical company jacked up the price of an antiparasitic drug. He later surrendered the album after his conviction on multiple counts of securities fraud in 2017.

In a statement, the U-S Attorney's Office for the Eastern District of New York says the proceeds from the sale will go towards the $7.4 million that Shkreli owes as part of his sentence.