According to his testimony, Robert M had an altercation with the victim after a drinking session in 2002 in the village of Lugi, also in northwestern Poland.
Having got into a fight, Robert and his friends took the man to the edge of a nearby lake, according to the testimony. Robert had allegedly then told Zbigniew, "You know what to do".
"Zbigniew B sat down on the man and cut his throat, then cut off his head, took off his clothes, gutted his body and cut off five pieces of his soft tissue from the rest of his body," Judge Tomasz Banas said while summing up the case. The flesh was then put on skewers and cooked over an open fire.
When one of the men protested Robert said he would suffer the same fate, the court heard.
The remains of the body were then dumped in a lake but in one of the twists that has puzzled investigators for decades, who the victim was and where he came from remains unknown.
He fits no missing persons records, no one has come forward to provide a name and no remains were ever found in the lake.
Another factor is that Rafal O has a long history of alcohol abuse and reportedly suffers from a mental illness - a point that was not lost on Robert M's defence team.
Monika Widacka, his defence lawyer, said they would appeal, saying the conviction was based upon the "testimony of one person who testified many years later, had alcohol problems and has been diagnosed with a mental illness".
On hearing the verdict, Robert M, who has always denied having any role in the murder, said: "I certainly did not expect this. I'm surprised."
Charges of desecration of the body have expired under the statute of limitations, leading to only the main instigator, Robert M, being sentenced.