The victim told investigators his ordeal began when he met Allen on a website named The Eunuchmaker, with Allen claiming 15 years experience in surgery.
The pair arranged for surgery to take place on October 12 and the victim travelled to Oklahoma from his Virginia home, The Smoking Gun website reported.
The victim reported being fully conscious as he was emasculated, receiving only local anaesthetic.
As he lay bleeding on the makeshift table, Allen reportedly told his victim he was a cannibal and was going to consume the body parts.
Court records show the victim told police Allen claimed to have "six more clients on the way to have the same operation," and that he "had a freezer with body parts."
Allen also allegedly shared that he "worked on someone that he described as crazy and that he left the male opened up to die overnight".
When he was still bleeding the morning after the surgery, the victim asked for help, only to be told: "No ER, no morgue".
The victim told police that after Allen relented and agreed to drive him to hospital he told him his body would be dumped in the woods if he died en route.
Allen pleaded no contest to felony counts of conspiracy to commit unlicensed surgery, practising medicine without a licence and attempted distribution of a psychedelic mushroom. He also pleaded no contest to two misdemeanour counts and was sentenced to a long stretch of imprisonment.
Allen told the judge he believed the plea was in his best interest because "a jury might convict me," the newspaper reported.
- with AP