In May 2014, Weier—then just 12 years old—and her friend Morgan Geyser lured a third friend, Payton Leutner, into the woods and stabbed her 19 times. Leutner survived, and when Geyser and Weier were subsequently arrested, they coldly explained to detectives that they were sacrificing Leutner to "Slenderman."
Slenderman is a Creepypasta meme along the lines of Anansi's Goatman Story and the tale of the Russian Sleep Experiment—a fictional horror monster dreamed up in message boards and Reddit posts by social media Lovecrafts. Weier and Geyser claim that they attempted to murder Leutner out of fear that Slenderman would come after them and their loved ones.
HBO covered the attempted murder and the subsequent trial in the true crime documentary, Beware the Slenderman, which premiered earlier this year.
Weier's guilty plea was accepted by the court, and her case will now be put in front of a jury to determine if she's guilty "by reason of mental disease or defect." She's now facing ten years in prison or three in a mental institution, depending on the jury's verdict, according to ABC.
"I believed that if I didn't go through with it, Slenderman would come and attack and kill myself, my friends, and my family. Those I cared about the most," Weier said Monday.
Morgan Geyser, who pleaded not guilty by reason of mental disease, is set to go to trial on October 16.