Waukesha County Circuit Judge Michael Bohren ruled on Aug. 10 that the trial of two girls accused of stabbing a classmate to please the online horror character Slender Man will stay in adult court. (Mike De Sisti/Milwaukee Journal Sentinel via AP)

Two 13-year-old girls charged with stabbing one of their classmates as a sacrifice to “Slender Man” — an infamous, fictional Internet myth — each pleaded not guilty to attempted homicide on Friday. The two teenagers will be tried as adults.

Morgan Geyser and Anissa Weier were both 12 when, authorities say, they lured their classmate into a wooded Wisconsin park and ambushed her. The victim was stabbed 19 times, and left for dead. The victim survived the attack.

(The Washington Post is naming the juvenile defendants since they will be tried as adults.)

The girls later told police that they believed “Slender Man” would reward them for their sacrifice by letting them live with him in his mansion, which they believed was located in the Nicolet National Forest in Wisconsin, about 300 miles away from their home, as the AP noted.

“Slender Man,” as The Washington Post has previously explained, is an online fictional villain usually depicted as an unnaturally tall, faceless man.

[The complete, terrifying history of ‘Slender Man,’ the Internet meme]

Waukesha County Circuit Judge Michael Bohren ruled earlier this month that Geyser and Weier would be tried as adults. 

“They’ve committed an offense that was serious. It was frankly vicious. It was a premeditated attempt to kill someone. There has to be assurance that that doesn’t happen again,” Bohren said, according to ABC affiliate WISN.

If convicted as adults, the teenagers would stay in a juvenile facility until age 18, and then serve out the rest of their term in an adult prison. Each teenager is charged with attempted first-degree intentional homicide, which carries a maximum sentence of 45 years in prison, the Milwaukee Journal Sentinel notes.

Defense attorneys argued that the teenagers would not get the mental health treatment they needed in the adult system. Bohren held separate hearings this summer to determine whether Weier or Geyser’s cases should move to the juvenile system, where they’d face substantially shorter sentences.

[‘Slender Man’ recedes in Wisconsin stabbing case as mental health becomes issue]

Weier’s attorneys argued that the 13-year-old was remorseful, wanted to get better and had the support of her family to do so, the Journal Sentinel reported.

Geyser was diagnosed early-onset schizophrenia, but has not been treated for her disorder, a psychologist testified during her hearing. According to the Journal Sentinel, Geyser has resisted taking medication for her symptoms “because it might make her “friends” — fictional characters from Harry Potter and Slender Man — disappear.”

After entering their pleas on Friday, the judge set an October 15 trial date for the case.