The chilling true story of "Roland Doe", the boy who inspired The Exorcist.

It’s the cult horror film boasting a nightmare-inducing scene which has remained etched inside the memories of many for years.

We’re of course talking about the moment a possessed 12-year-old girl’s head spun 360 degrees in The Exorcist, and we’re deeply sorry for reminding you of it.

(There was also that scene with the crucifix, but let’s leave that in the past, shall we?)

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The 1973 film terrified a generation, and to this day remains one of the scariest horror films of all time.

But while most of us convinced ourselves that the film was simply fiction after we first watched the movie, there’s actually a chilling true story behind the film.

A story about a troubled 13-year-old boy named Ronald, whose experiences are believed to have inspired the 1971 novel by William Peter Blatty, which went on to form the basis of the film.

It all began in the 1940s, behind the front door of a handsome red brick Colonial-style house - a home which remains a landmark for horror fans.

Ronald Hunkeler, who later became known in literature as Roland Doe, was mourning the loss of his Aunt Harriet, a spiritualist who taught him about the supernatural, when he began hearing scratching sounds coming from the floors and walls of his bedroom.

He started screaming as though in agonising pain for no discernible reason and marks appeared on his body as if scratched into his skin by an invisible force. Water dripped from his bedroom walls, and sometimes, his mattress would violently move without explanation.

Ronald's disturbed family rallied to help their son, enlisting doctors, psychiatrists and their local Lutheran minister, none of whom could offer an explanation or remedy for his strange behaviour.

Eventually, they sought assistance from the local Catholic priest, Father E. Albert Hughes, who was convinced the boy was possessed and in need of an exorcism to remove the demonic energy from his body. However, Hughes was forced to stop the process short when Ronald tried to slash him with a piece of spring from the mattress, wounding him on the shoulder.

Days later, an alarming series of red scratches appeared on the boy - one which formed the word 'LOUIS', a sign which Ronald's mother took to mean St. Louis, where they lived in their red brick home.

This is when the Jesuit church stepped in; specifically Father Walter H. Halloran and Rev William Bowdern.

As reported by the Daily Star, before he died in 2005, Father Halloran recalled meeting the young boy when he was just 26.

"Before we started prayers we had a bottle of holy water. I put it on the dresser and was kneeling there at the front of the bed and all of a sudden this bottle whizzes past my head and crashed into the wall," he said.

"That was the first clue that I had that this was a case of exorcism. I’m convinced that the situation of Roland and the things he went through was all genuine, there was not any deception on his part."

Father Halloran and Rev Bowdern, along with several helpers, agreed to exorcise Ronald in March of 1949.

According to their reports, amid Ronald's usual display of screaming and wild outbursts, scratchings on his body and the mysterious moving mattress, they noticed a pattern. While he seemed calm during the day, the strange behaviour began each night when he was preparing to go to sleep.

They wrote that he would enter a trance-like state and start making sounds in a guttural voice, and that objects would eerily float around the room in his presence. On top of all this, he was angered by any sacred object presented by the Jesuits - lashing out at the men if they attempted to move closer to his bed.

At one point during the ordeal, Bowdern claims he saw an “X” appear in scratches on Ronald’s chest, which the priest believed signified the number 10 - the number of demons that had possessed the boy.

In another incident, a pitchfork-shaped pattern of red lines moved from the boy’s thigh and snaked down towards his ankle.

Similar events occurred every night for more than a month while the priests continued the exorcism, until one night, it reached terrifying new heights.

On the evening of March 20, Ronald urinated all over his bed while cursing and screaming at the priests that "Satan would always be with him".

It was the final straw for his family, who took him swiftly to Alexian Brothers Hospital in St. Louis for treatment.

However, the episodes continued, night after night, until April 18 - the Monday after Easter. Ronald had awoken with seizures, so the priests lay holy relics, crucifixes, medals and rosaries on his body in a last ditch effort to remove the evil energy, calling on St. Michael to expel the devil from his body.

Suddenly, after months of terrifying displays, in what the family called a "miracle", the seizures stopped and Ronald simply said "He's gone".

From that night forward, Ronald went on to lead a completely normal life, with no further documented examples of strange behaviour.

The exorcism of "Roland Doe" came to light in a 1949 article in The Washington Post, before the novel by William Peter Blatty was penned based on unofficial diaries kept by Halloran and Bowdern in 1971.

The book, which stayed on the bestseller list for 54 weeks, led to the creation of the movie (and basis of nightmares) in 1973.

Ronald was swapped out for a 12-year-old girl named Regan, and although the scratches, shouting, spitting, red lines on the skin, and cursing in the movie mirrored Ronald's tantrums, the boy’s head never turned 360 degrees like Regan’s did.

Nevertheless, it sounds just as terrifying in real life as what was portrayed in the film.