Yesterday in rural Wisconsin, two 12-year-old girls allegedly stabbed a friend of theirs 19 times and left her to die in a forest. The girl survived, but the reason for her attack remains puzzling. Her accused attackers apparently told police that she was meant to be a sacrifice to a mythical creature popular in the genre of internet-based horror fiction known as "creepypasta"—a character called "The Slender Man."

Hold up: Internet-based horror fiction?

The character that eventually became so important to two girls that they stabbed their friend started on the forums of the humor pit Something Awful. According to the reliable web archive Know Your Meme, Slender Man first appeared in 2009 in a series of images created by a man named Eric Knudsen, who entered two photos into a "paranormal pictures" Photoshop contest. Know Your Meme describes the character thusly:

Slender Man (a.k.a Slenderman) is a mythical creature often depicted as a tall, thin figure wearing a black suit and a blank face. According to the legend, he can stretch or shorten his arms at will and has tentacle-like appendages protruding from his back.
The character quickly became popular on the thread, with other posters making their own Slender Man images while also fleshing out his backstory. Over the years, The Slender Man trickled out to other forums and took on attributes and a personality like any true horror villain.

The Slender Man wiki sketches out the two main characterizations:

In total two major types of Slender Man descriptions exist. The original versions depict him as not only malevolent, but extremely dangerous. He will do odd things such as removing organs and placing them in bags, impaling targets on trees, and aggressively stalking targets after a period of time. A contemporary depiction shows Slender Man as being extremely passive aggressive, often letting a person slowly delve into madness at their situation until being unable to cope. If invited to anger, he will typically charge a target down until caught and vanish with them to unknown locations. This version also shows an odd response to electronic equipment and causes massive problems with audio, surveillance, camera, and other various electronic devices with displays or audio. Sometimes an individual can determine if he is near simply by how certain electronics react such as radios, televisions, or cameras.
Knudsen, in an interview with a Slender Man fansite (linked to by by the Daily Dot), correctly tabbed The Slender Man as "an accelerated version of an urban legend." That legend has accelerated all the way up to film, where The Slender Man has become the focus of web shorts, a YouTube series and two feature-length films. One film is titled The Slender Man, and it was funded by $11,012 raised via Kickstarter in 2012.

But the most well-known adaptation of The Slender man is that YouTube series—the sprawling Marble Hornets. The series was created by a Something Awful poster named Troy Wagner, who posted the first "entry" on June 20, 2009, just a few weeks after Knudsen invented the character. He is now on entry number 85—posted just a few weeks ago—and last December Wagner said that he had finished shooting a full-length film adaptation of the series directed by the horror veteran James Moran and starring the actor Doug Jones, who you may know as the guy in Pan's Labyrinth with eyeballs on his palms.

Despite The Slender Man character being elevated to some of our most easily accessible mediums, the Wisconsin girls are said to have stumbled on the character via the wiki for Creepypasta, a database for horror stories of the same name. (Says the wiki: "a creepypasta is a short story posted on the Internet that is designed to unnerve and shock the reader.") There are hundreds of Slender Man stories on Creepypasta, most of which seem to focus on the protagonist being hunted by the tall, cloaked, shadowy figure.

One, written in 2012 by a woman named Raksha Keller, is titled "Slenderman Doesn't Exist." It ends with this sentence:

Slenderman doesn't exist. Sure. But you do. As do the monsters inside….