They became obsessed with an online character. And so they decided to kill their friend.

WARNING: This post has details of a violent crime and may be distressing for some readers.

It is an unusual background image for a 12-year old girl’s iPad – that of a mythical online bogeyman called the ‘Slender Man’.

It is of a thin, unnaturally tall man with a blank and featureless face wearing a black suit.

The Slender Man.

Said to stalk, abduct, or traumatise people – mainly children.

This particular character features heavily on the website Creepypasta, where people share their own short stories about the paranormal.

For two 12-year-old girls the fictional Slender Man was so real they were willing to kill to appease him.

These two 12-year old girls are described as being normal, and as being “so nice they would not even give a paper cut to someone”.

But they were obsessed with Creepypasta – and eventually they came to believe that they needed to sacrifice blood in order to appease Slender Man. To enter into his world, they needed to murder someone. So they planned it.

For months the pre-teens from Milwaukee, Wisconsin plotted every detail secretly.

They planned to kill a close friend and then move to the Nicolet National Park, where they thought Slender Man owned a mansion. They were so wrapped up in this online fantasy world that they never seemed to stop for a minute to see it was just that – a fantasy.

Their first plan, according to Prosecutors, was to gag their friend as she slept before stabbing her in the neck as she lay in bed.

CCN reports that they told police they thought a bathroom would be a better scene for the crime, because it would be easier to clean up after.

After consulting each other, the girls ditched their original plans and instead decided to kill their close friend in the woods following a game of hide-and-seek. The victim had a sleepover with the two girls on a Friday night — before they stabbed her on Saturday.

Police say they walked with her to a park. Court documents say that: “As they left for the park … (the victim) was walking in front of them and (the 12-year old accused) lifted up the left side of her white jacket and displayed the knife tucked in her waistband.”

“The accused stated she gave the other girl a look with wide eyes and, when asked what that meant she stated, ‘I thought, dear god, this was really happening.’”

The victim was taken to the woods and then stabbed 19 times.

She crawled from the woods before being rescued by a passer-by.

During police questioning, one of the 12-year old girls told investors that she regretted the stabbing, the Milwaukee Journal Sentinel reported. “The bad part of me wanted her to die, the good part of me wanted her to live,” she told police.

According to The Sydney Morning Herald a forensic psychologist said these types of actions were very rare.

Nancy Kaser-Boyd from the Geffen School of Medicine at the University of California, Los Angeles said typical 12-year-old children know that killing a person is a permanent action, but that they might not fully understand the consequences.

“By the age of 12 they certainly have a basic understanding of right and wrong but they can get caught up in a fantasy world,” she said. “Their thinking is not yet mature.”

Similar to a mentally ill adult who suffers from delusions, these girls engrossed in a fantasy world may have believed they were justified in the attempted murder.

The lawyer for one of the girls has said that he believes she is mentally ill.

The two girls were charged with attempted first degree murder as adults, thus why they have been named in some articles and their images released.

If convicted, they could face up to 65 years in prison.