Anneliese Michel was a German woman who is believed to have experienced a true demonic possession. At the age of sixteen, Anneliese had an epileptic fit and developed temporal lobe epilepsy. She also suffered from depression and was treated at a psychiatric hospital, where she first reported that she was hearing voices. Her family, who had a strong Catholic faith, were convinced she was possessed and demanded a priest to perform a controversial exorcism. Over a period of years, 67 exorcisms were performed, despite her clearly needing medical help. Wounded and neglected, she eventually died in 1976 from starvation and dehydration. Two priests were charged with negligent homicide. 

The eerie audio of her exorcism can be listened to here.