Mark Moody was an insane serial killer who was once imprisoned at the Green River County Detention Center until his death.
Moody was a prolific serial killer who was deep into Satanism and ritual murder and was described as a lunatic. After being captured, Moody was incarcerated at the Green River County Detention Center. One night, the guards attacked Moody and bashed his head in, killing him.
Following Moody's murder, his death was reported as a heart attack. His cell block mate Randall knew the truth, but kept silent out of fear that he would suffer the same fate. Moody's brutal death left a massive amount of blood in his cell which Randall was forced to clean up the next day. There was so much blood, that it all couldn't be cleaned up, particularly on Moody's mattress. Moody's body ended up being cremated, leaving the bloody mattress his only form of remains. Shortly after Moody's murder the cell block was closed down and the mattress with Moody's blood on it was left abandoned in the cell block.
In 2007, after the cell block was reopened a ghost was released that started killing prisoners and corrupt guards. The warden, Deacon, contacted hunters Sam and Dean Winchester who suspected Moody as the culprit due to him dying in the old cell block of a "heart attack" right before it was closed up and the victims dying of heart attacks as well.
After getting themselves purposefully arrested, the Winchesters dug into what could be keeping Moody tied to the prison as he had been cremated. Sam eventually met Randall who was in the prison once again and had witnessed one of the murders. Randall informed Sam of the truth of Moody's death and the Winchesters came to suspect that the leftover blood was what Moody was attached to. As Dean created a distraction, Sam managed to sneak into Moody's old cell and salt and burn the mattress which the Winchesters believed dispatched Moody's ghost and ended the threat.
Shortly after the salt and burn, Tiny was murdered in front of Dean by the real ghost, a prison nurse. With the help of Randall, the Winchesters identified the real ghost as Nurse Dolores Glockner who was murdered by inmates in the old cell block shortly after Moody's death.