Slender Man in background
Slender Man in the background of one of the many doctored images that have gone viral since 2009Bob Mical/Flikr

Slender Man, or Slenderman, lurked in corners of sepia photographs. His tall, spectral figure is sombre and terrifying, just like the rumours that shroud his character in mystery. While some say he lurks in woods to prey on children, others claim he stalks dimly-lit streets at night to hunt unfortunate humans.

Disturbingly, Slender Man has no face. But some say his face appears differently to whoever sees it. Named the "first great myth of the web" by journalist Aleks Krotoski, the Slender Man is the stuff of nightmares - wrapped in an ill-fitting suit.

The urban myth percolated the real world with terrifying consequences this week, as news broke that two 12-year-old girls has been charged with attempted murder for stabbing a classmate - allegedly in order to please the fictional online character.

Slender Man with girl
One image of Slender ManCreepypasta

Anissa Weier and her friend Morgan Geyser lured their friend to woods in their home town of Waukesha, Wisconsin and stabbed her 19 times, in a plan inspired by a story the pair had read on a website called Creepypasta. The girl survived, thanks to the intervention of a passing cyclist.

Charged with first-degree attempted homicide, the girls could face up to 60 years in prison. They reportedly claimed that they wished to commit a murder as the first step to becoming acolytes of the Slender Man.

In the myth, Slender Man has several origins, including Germany's Black Forest, ancient Egypt and cave paintings in Brazil.

In reality, the Slender Man was created in a thread on the Something Awful internet forum in 2009, to be edited into photographs for a supernatural effect. But the myth of Slender Man snowballed when one user, known as Victor Surge, juxtaposed the eerie photographs with snippets of text describing alleged abductions of groups of children.

One read: "One of two recovered photographs from the Stirling City Library blaze. Notable for being taken the day which fourteen children vanished and for what is referred to as "The Slender Man". Deformities cited as film defects by officials. Fire at library occurred one week later. Actual photograph confiscated as evidence... 1986, photographer: Mary Thomas, missing since June 13th, 1986."

The Slender Man soon went viral, spawning cosplay, fanart and Creepypasta fiction. The myth was even used in a video series called Marble Hornets, published in the found-footage style on YouTube, by Troy Wagner.

Other Slender Man-inspired works include a BBC 4 documentary called "Slender Suits". In 2013, it was announced that Marble Hornets would be turned into a film.

In 2011, Markus Persson, who created the game Minecraft, added a character to the game called Enderman - which Reddit and Google + users linked directly to the Slender Man. In 2012, the character was adapted into a video game called Slender: The Eight Pages.

Surge, creator of Slender Man, said in an interview with Know Your Meme: ''I didn't expect it to move beyond the SA forums."

Creepypasta released a statement after the stabbing earlier this week, offering condolences to the families affected.

"I'm going to make myself loud and clear: ALL WORKS PRESENTED ON THIS WIKI AND OTHER SITES (INCLUDING SLENDER MAN...) ARE FICTIONAL STORIES AND CHARACTERS," a site administrator known as Sloshedtrain wrote.

"This wiki does not endorse or advocate for the killing, worship, and otherwise replication of rituals of fictional works. There is a line of between fiction and reality, and it is up to you to realise where the line is. We are a literature site, not a crazy satanic cult."