Slenderman, according to Fox 6 Now in Milwaukee, is the leader of CreepyPasta, a website that talks about and archives horror stories. The Slenderman entry in wikipedia describes the ficticious work as not tied to any particular story, but appearing in many disparate works of fiction, mostly composed online.
Proxies, according to a website dedicated to Slenderman, "do the actual, physical work for Slender Man, such as creating and manipulating objects, destroying and leaving evidence, creating videos and responding on Twitter, and influencing victims as needed."
According to the criminal complaint, one of the accused girls claimed they had to kill someone to show dedication to the site's leader.
Police said the girls lured the victim to a wooded area, stabbed her 19 times, and left her to crawl to her own rescue WGN reported.
Police said a bicyclist found the girl alive and she was hospitalized Monday.
The Associated Press reported one of the stab wounds missed a major artery near her heart by a millimeter.