(CNN) -- 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 WNYC's "TLDR" podcast. "But I don't think anyone really expected that to happen."
Other Something Awful users began creating their own Slenderman stories. And they spread to other sites. Over the past five years, he's appeared in fan art, short stories, videos, video games and other media all over the Web.
A Google search for "Slenderman" on Tuesday returned more than 4 million results.
"Marble Hornets" is one of the most popular permutations. It's a video series on YouTube, presented in "Blair Witch Project"-like faux-documentary style, with nearly 380,000 subscribers.
Slenderman has also been a popular subject of "creepypasta," a form of Web-based short fiction. A play on the term "copypasta," which itself is derived from the keyboard action "copy-paste," creepypasta is horror fiction written with the Web in mind and, often, done in a style that makes it appear like a news item or other piece of true crime.
The girls accused in the Milwaukee stabbing told police they knew the character from the Creepypasta Wiki, a site that compiles such fiction. The site has issued a statement condemning the attack.
"This is an isolated incident, and does (not) represent ... the Creepypasta community as a whole," the statement reads. "This wiki does not endorse or advocate for 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 realize where the line is. We are a literature site, not a satanic cult."
Knudsen, too, has lamented losing control of his creation.
"I feel like less of a creator and more of an administrator, in a lot of respects, or the manager," he told NPR in the interview from January. Knudsen, who could not be immediately reached for comment, has filed for a copyright on the character.
"I feel like I'm Slenderman's manager, and he's out there doing his thing, and I need to just kind of watch him and take care of him."