Slender Man with girl
One image of 'Slender Man': a modern day fairy tale monster.(Creepypasta)

A 12-year-old girl accused of stabbing her friend to appease the bogeyman Slender Man has been declared unfit to stand trial.

A judge the ruling about Morgan Geyser, after hearing reports from psychologists that she believed that she could speak to fictional characters, including the Harry Potter villain Voldemort.

Geyser and Anissa Weier, 12, have both been charged with attempted homicide, after luring their friend into woodland near Milwaukee before stabbing her 19 times.

The victim was left bleeding in the woods, but was able to crawl out and was found by a cyclist.

They told police that they had stabbed her to please the Slender Man, who they believed was real.

Their victim, a 12-year-old who has not been named, was hospitalized for six days by her wounds.

The judge ruled that Geyser should be committed to Wisconsin's department of health services, after two psychologists testified that she was unable to assist in her own defence.

Psychologist Brooke Lundbohm said Geyser told her that she sees unicorns, she believes she has mind control powers, and believes Harry Potter villain Voldemort told her that Slender Man is real.

She said: "[Geyser] presently lacks substantial mental capacity to rationally and factually understand her charge and be a meaningful assistance in her defense."

Psychiatrist Kenneth Robbins said the girl's main concern was her relationship with Slender Man, and she was unconcerned about the prospect of a long prison sentence.

He added: "She does have concern about angering Slender Man and so she has to be careful about what she says."

The court heard that Geyser could become fit for trial, depending on her growth, and the treatment for her psychological disorder.

Geyser is next due to appeal in court on 12 November, while her co-accused Weier is due to appear in September.

The girls face up to 60 years in prison, if convicted of attempted homicide as adults.

They could be incarcerated to age 25 if they are convicted as juveniles under Wisconsin law.

The Slender Man originated a competition to create a scary story in internet forum Something Awful.

He went on to feature as a bogeyman character in online fiction and web TV series, in which he stalks and abducts people, especially children.

Slender Man is depicted as an unnaturally tall, thin man, with no facial features, wearing a dark suit.

A woman, who did not want to be identified, told WKWD TV in Ohio that her 13-year-old daughter, who had written macabre fiction inspired by Slender Man, had attacked her with a kitchen knife.

The Marble Hornets web series follows a filmmaker who becomes the prey of Slender Man.(MarbleHornets/YouTube)