The girls were aged 12 when they allegedly attacked a friend of the same age after a slumber party, stabbing her 19 times and leaving her in the woods in Wisconsin.
Miraculously Payton Leutner survived and managed to crawl to a nearby road where she was found by a cyclist who raised the alarm.
Scroll down for a gallery of Slender Man pictures
Slender Man was created in 2009 as part of an online competition calling for scary images
The suspects have claimed they were trying to prove the existence of a fictional horror character called “Slenderman”.
They have both been charged with attempted first-degree intentional homicide in the attack in May in Waukesha.
Prosecutors say the girls plotted for months to kill their classmate, luring her to a wooded park after a sleepover and stabbing her repeatedly.
The pair were found walking toward a national forest where they said they believed Slenderman lived in a mansion.
All three girls were 12 at the time, though one of the suspects has since turned 13.
Slender Man is said to stalk children
The judge ordered one girl to undergo mental health treatment in August after a psychologist testified that she claimed to see and hear unicorns, Slenderman and Lord Voldemort, the villain from the "Harry Potter" series.
Psychiatrist Kenneth Casimir however reported that her mental state had improved.
Her attorney has informed the judge his client suffers from schizophrenia, but declined to challenge Casimir’s report.
The judge set a joint preliminary hearing, the point in the legal process where he will have to decide whether enough evidence exists to proceed to trial, for February.
The Associated Press isn't naming either defendant because their attorneys have said they may still try to move their cases into juvenile court, where proceedings are secret.
Slenderman has also been linked to the stabbing of a woman by her 13-year-old daughterand the murders of two Las Vegas police officers.
According to Know Your Meme, which tracks internet trends, Slenderman came into being in 2009 after an entry for a competition on the online forums Something Awful to create “paranormal pictures” was submitted by Eric Knudsen.
The popularity of the character promptly exploded, with numerous fan sites creating their own Slenderman stories, putting him in fan art, videos, video games and other media. Some websites host what appear to be photographs of him, lending the creation an air of authenticity.
Shira Chess, an assistant professor at the University of Georgia who has researched Slenderman's origins told the Associated Press: "It feels real. A 12-year-old isn't potentially going to know the whole origin of the story."
Knudsen’s creation appears to be a mysterious figure who stalks children.
He is most distinctive for his blank, featureless face, his extreme height and dapper appearance – (“He often keeps his long, pale hands crossed politely behind his back or hanging loosely at his sides. He has long coattails which he lets flow proudly. He wears long dress shoes, which are always shined a perfect, gleaming black”).
According to some descriptions, he has between four and eight long black tentacles, which protrude from his back. He is also said to be capable of causing memory loss, insomnia, paranoia and coughing fits.
The girls in this case reportedly told detectives they learned about Slenderman on online fiction forum Creepypasta Wiki.
Literature on the site reads: “Whether he absorbs, kills, or merely takes his victims to an undisclosed location or dimension is also unknown as there are never any bodies or evidence left behind in his wake.
“His face is pale and slightly ghostly and almost appears to have been wrapped in a type of gauze or cloth. His facial features are also an object of debate and many people believe that his face looks different to each person, if it is seen at all.”
The site has distanced itself from the Wisconsin stabbings, issuing the following statement to CNN: "This is an isolated incident, and does (not) represent ... the Creepypasta community as a whole.
"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 realise where the line is. We are a literature site, not a satanic cult."
Claire Lilley, Head of Child Online Safety at the NSPCC told Huffington Post UK via email: “Things are not always what they seem online and children can get sucked into unhealthy and dangerous situations.
"While the web is a wonderful resource it also harbours risks. Parents should talk to their children about what they are viewing.
"And it's vital youngsters know who to turn to if they ever feel threatened or frightened by any online encounter. This way we can hopefully avoid more tragedies like this.”