He lurks in the background of gritty black-and-white photos - a gaunt, too-tall figure with skeletal limbs.
Some say he lives in the woods and eats children, a kind of demon descended from eastern European myth.
Some say he stalks human prey indiscriminately, wherever he can find it: in basements, outside half-open windows, along lonely streets late at night when only occasional headlights cut across the road.
Some say he has no face. Others, that his face looks different to everyone who sees it.
Whatever they say, though, everyone generally agrees on two points: that Slender Man may be the internet's best and scariest urban legend, and that he is utterly fictional.
But that did not stop his legend almost resulting in the death of a 12-year-old girl in the United States, when two other girls her age plotted to kill her to curry favour with the character.
On Saturday, a cyclist found the young victim in the middle of a path in Waukesha, in the state of Wisconsin. Her clothes were caked with blood from multiple stab wounds.
Morgan Geyser and Anissa Weier, both 12, have been charged as adults with attempting to murder their friend and classmate after inviting her for a sleepover on Friday night.
The two girls told investigators they plotted for months to kill their friend because a Slender Man required them to kill someone before they could become his "proxies".
Both girls are being held at a juvenile detention centre. Anthony Cotton, the lawyer for one girl, said a judge rejected his request to have her transferred to a mental health facility, but said he would renew the request.
The girls told investigators they read about Slender Man on the horror website creepypasta .wikia.com 
The plot was months in the making, police said. After luring their friend into the woods, one of the girls held the victim down while the other stabbed her in the arms, legs and torso. After her assailants left, the victim was discovered. She was in stable condition, police said, but had to undergo surgery for her injuries.
The case prompted Waukesha police chief Russell Jack to warn the other parents in his community near Milwaukee.
"The internet has changed the way we live. It is full of information and wonderful sites that teach and entertain," Jack said. "The internet can also be full of dark and wicked things."
According to the Creepypasta website, "a creepypasta is a short story posted on the internet that is designed to unnerve and shock the reader".
But the girls were apparently taking the website much more seriously than that. Anissa told police that Slender Man was the "leader" of Creepypasta, and that she and Morgan wanted to become his "proxies" by killing someone.
As best as anyone can tell, Slender Man was first created on a general-interest internet message board as part of a Photoshop contest in 2009.
Slender Man has since become a staple in the gothic corners of the internet.
Web advocates find themselves on the defensive. "I'm going to make myself loud and clear: ALL WORKS PRESENTED ON THIS WIKI AND OTHER SITES (INCLUDING SLENDERMAN) ARE FICTIONAL STORIES AND CHARACTERS," a site administrator wrote on Tuesday.
Washington Post, McClatchy-Tribune, Associated Press