For years now, the internet boogeyman known as Slender Man has been haunting the online world, cropping up in short stories, fan art, and any number of popular short films. The most upsetting thing to come from the character—a faceless, tall man in a suit, which has its origins on the Something Awful message boards—isn’t a particular meme or YouTube clip, though. Instead, it’s a 2014 event in which a young girl barely survived being stabbed by two of her 12-year-old classmates, supposedly at the creature’s behest.

Dubbed “the Slender Man stabbing” due to the two girls’ belief that the internet meme had ordered them to kill their friend, the case is still making its way through the Wisconsin courts. Now, HBO has premiered the first trailer for Beware The Slenderman, its upcoming documentary about the case, and the hysteria surrounding the online character. And while the short promo is full of jump scares and spooky tentacles, the most chilling moments have nothing to do with monsters, and everything to do with the human actions and beliefs at the center of the crime.

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[via Buzzfeed]