Slenderman Stabbing

He’s the Internet’s own monster, a ghoul who lurks in its darkest corners and, like the Web itself, has mutated time and again to suit the dreams and desires of his devotees.

He is Slenderman, a menacing, faceless specter in a dark suit — sometimes portrayed with octopus-like tentacles — known to haunt children and those who seek to expose him. He was born in 2009 in an online forum for people who enjoy creating fake supernatural images.

And, on Saturday, police say, he played a role in the attempted murder of a 12-year-old girl in suburban Milwaukee by two female classmates who stabbed her 19 times. According to police, the girls said the attack was meant to impress the fictitious bogeyman.

To be clear, the origin story of the monstrous character (sometimes referred to as The Slender Man) in no way urged readers to kill to earn his favor. But Slenderman has undergone hundreds of permutations online in his five-year existence.

In June 2009, a Photoshop contest for images that appeared to be paranormal was launched in a forum on the website Something Awful. According to Know Your Meme, a blog that chronicles Web culture, the goal of the contest was to create the images and then use them to fool, or “troll,” other Web users by submitting them to paranormal websites.

Site member Eric Knudsen (under the screen name “Victor Surge”) submitted two images to the contest, both black-and-white images of children, one of which appeared to show a largely undefined figure lurking in the background.

They were presented as being from 1984, and one included the text ” ‘We didn’t want to go, we didn’t want to kill them, but its persistent silence and outstretched arms horrified and comforted us at the same time…’ — 1983, photographer unknown, presumed dead.” (Know Your Meme has documented these posts, although links to the original thread no longer work.)

A day later, according to Know Your Meme, Knudsen added a third photo and a fictional doctor’s account of a mass killing. And, from there, Slenderman’s assault on the Internet began.

“Some people joked in the thread, ‘Wouldn’t it be funny if some of these ended up on those paranormal websites or someone said these pictures look real,’ ” Knudsen said in an interview this year with National Public Radio’s “On the Media.” “But I don’t think anyone really expected that to happen.”