But the creature has since become a popular internet horror figure, and many within the online horror community have built on his origins, and spun a number of related stories, legends and fan art about his supposed activities.
The girls reportedly said they learned about the creature on creepypasta.wikia.com, a website about horror stories and legends. Here, the creature is described as a thin man, dressed in a black suit, with a ghostly pale face (usually with no visible facial features), extremely long slender arms and legs, and has long black tentacles protruding from his back.
He apparently has the ability to stretch his limbs and torso and put his victims in "something of a hypnotized state, where they are utterly helpless to stop themselves from walking into them," creepypasta.wikia.com says.
According to the website, Slender Man's "main intentions appear to be kidnapping children, as when he is seen near them in photographs, they usually disappear shortly afterwards."
"Whether he absorbs, kills, or merely takes his victims to an undisclosed location or dimension is also unknown as there are never any bodies or evidence left behind in his wake to deduce a definite conclusion," the website says.
Seen mostly at night
He is said to live in the woods and preys on children and has also "been reported seen with large groups of children, as many photographs portray."
"He is seen mostly at night peering into open windows and walks out in front of lone motorists on secluded roads," creepypasta.wikia.com says.
According to the criminal complaint, the Wisconsin girls planned to run away to Slender Man's forest mansion after the slaying. One girl told police they decided to kill their friend so they could become proxies of Slender Man, who would accept them and let them live with him in his mansion in the Nicolet National Forest.
The other girl said she sees Slender Man in her dreams. She said he watches her and can read her mind and teleport.
The creature is said to be the brainchild of Eric Knudsen, who created Slender Man under the username "Victor Surge," as part of a 2009 Photoshop contest held by the Something Awful Forums website.
The contest asked participants to take real ordinary-looking pictures, add in a paranormal element, and then pass off those pictures as real on paranormal forums, according to knowyourmeme.com
Knudsen posted two black and white photographs of real children with the Slender Man character superimposed in the background.
He described Slender Man as a mysterious creature who stalked children, according to knowyourmeme.com
"It was pretty spontaneous. I saw some of the pictures in the thread and just decided to make something that I myself would find creepy," he said in a 2011 interview posted on knowyourmeme.com.
Knudsen said he was influenced by H.P Lovecraft, the short stories of Stephen King and the surreal imaginings of William S. Burroughs. He said his most direct influences included Zack Parsons’s That Insidious Beast, Stephen King's short story The Mist and reports of so-called shadow people.
"I used these to formulate something whose motivations can barely be comprehended and causes general unease and terror in a general population."