In 2014, a scary internet fiction tragically overwhelmed reality when 12-year-olds Morgan Geyser and Anissa Weier stabbed a classmate in the woods after developing a fascination with Slenderman. The girls believed that it was necessary to prove that Slenderman, which began its life on the internet as a meme, existed, and that killing someone was the only way to do so. Fortunately, the victim survived, and the trial is still ongoing. The disturbing case is being explored in a true-crime documentary called Beware the Slender Man. The documentary, directed by Irene Taylor Brodsky, will air on HBO January 23. Watch its trailer above.