Slender Man: Horror character may have led two teenage girls to brutally stab a classmate. But who - or what - is he?

Until this week, Slender Man was known only to internet horror fans who would write stories about him and share his legend. But his tale took a sinister turn after it was revealed that two Wisconsin schoolgirls had allegedly stabbed a classmate because they wanted to please the mythological character

Slenderman was crafted as a lanky man, with a featureless face, sometimes with tentacles protruding from his back, who would stalk and abduct children before murdering them in the woods

There is a good chance that you've never heard of Slender Man, but online, where he was born and bred, he is huge. It is not the Bogeyman or Bloody Mary that scares children today, but a long-limbed, faceless man in a suit.

Until this week, Slender Man was known only to internet horror fans who would write stories about him and share his legend – albeit vast numbers of them; a Facebook page dedicated to the creepy creature boasts one million likes.

But his tale took a sinister turn this week after it was revealed that two 12-year-old schoolgirls in Waukesha, Wisconsin, had allegedly stabbed a classmate 19 times and left her for dead because they wanted to please the mythological character. The pair were reportedly fans of Slender Man and had grown obsessed with him over the past few months, plotting to kill their friend since December.

But who is Slender Man exactly, and where did he come from? And is he really any more ominous than other frightening fictions designed to keep us up at night?

An online search for Slender Man turns up "sightings" of him dating back hundreds of years. However, he was actually created in June 2009, and fans have merely drawn up an elaborate backstory.

The website Something Awful invited users to send in Photoshopped images that inserted the paranormal into everyday snaps. A user called Victor Surge (real name Eric Knudsen) uploaded two black-and-white photographs featuring groups of children and a figure that he called "The Slender Man". The pictures came with captions about the children in the pictures being found murdered and the photographers going missing. Knudsen had unwittingly created a web phenomenon.

Scare tactic: Slenderman images have attracted a huge following on Facebook

Slender Man was crafted as a lanky man, with a featureless face, sometimes with tentacles protruding from his back, who would stalk and abduct children before murdering them in the woods.

But as internet users began to create tales and contribute further manipulated images, different details began to appear. Some gave him the ability to teleport or stretch to extreme heights. Others believed that he could read people's minds or cause memory loss (it should be noted, however, that the mythos of Slender Man does not include him encouraging other people to kill).

The legend of Slender Man grew through creepypasta, the internet phenomenon that sees the spreading of scary stories told in snatches of copyable text that travel from site to site, with users being urged to pass on the terrifying tale, almost like a chain letter.

Knudsen keeps a low profile, but has expressed his surprise at how much the public has embraced his creation. In a 2011 interview with a Slender Man forum, he said dismissively: "I don't spend a lot of active time on the internet since I usually have a lot of real-life stuff going on."

Slender Man's ownership is unclear, and while Knudsen is often accredited as its creator, an unidentified third party reportedly owns the rights to media adaptations (such as a video game). A film posted to YouTube about Slender Man received 500,000 views in three weeks before it was removed two months later for copyright infringement.

Rescue workers take a stabbing victim to the ambulance in Wisconsin. Prosecutors say two 12-year-old girls stabbed their friend 19 times (AP)

The two Wisconsin girls apparently came across Slender Man on CreepyPasta.com (one of the many websites dedicated to the art of creepypasta), which describes itself as a site featuring "scary paranormal stories and short horror microfiction". On it, you will find tales on everything from aliens to zombies alongside the Slender Man files.

The anonymous creator of the site has now felt compelled to issue a statement about the stabbing, having seemingly received abusive messages. As well as offering condolences to the families of those affected, he or she insists (in a 4,000-word blog post) that the website cannot be held responsible for the stabbing. "Most people don't watch Hannibal and turn into serial killers," went the reasoning.

Despite the story of Slender Man being no different to that of any other horror creation, it looks unlikely that the statement will appease those looking for someone to blame for the incident in Wisconsin. And, so, the ongoing argument about the relationship between art and violence rages on.