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HBO released the trailer for its new documentary Beware the Slenderman, and it is terrifying.

The documentary—set to release January 23 —dives deep into the psyche of two 12-year-old girls who stabbed their classmate 19 times, an action inspired by their devotion to the faceless online bogeyman, Slender Man.

In 2014, two girls led their 12-year-old friend into the woods and stabbed her over and over again. The girls later claimed they committed the crime in order to prove the existence of Slender Man and so the fictitious creature wouldn’t kill their families.

Morgan Geyser and Anissa Weier have each been charged with attempted first-degree intentional homicide, and the long-running case is still pending. Their cases may not go to trial before next summer. Each has pleaded not guilty by reason of mental disease or defect— in other words, the insanity defense.

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Slender Man began as an internet urban legend of sorts. According to The Washington Postthe story began in the forums of Something Awful—basically a humor site for nerds (think Dungeons and Dragons groupies). On June 8, 2009, a new forum invited users to “create paranormal images through Photoshop,” and user Victor Surge jumped right in.

He posted two old photos of children haunted by a very tall, creepy figure with octopus-like arms.

The WP reports that for weeks afterwards, Surge continued posting newspaper clippings, doctored photos, and children’s drawings of Slender Man. Other users joined in, drawing in older legends and expanding the story even more. By mid-June, the thread was solely devoted to developing the myth of Slender Man, which—according to the Washington Post— now runs 194 pages long in PDF format.

The story of Slender Man was just vague enough to allow consumers to fill in their own blanks and add their own spooky details. BAM! An urban legend was born which seamlessly drifted into mainstream forums. Including the site Creepypasta where—according to the Milwaukee Journal-Sentinel via The Washington Post—the preteen girls were introduced to Slender Man.

The documentary was directed by Irene Taylor Brodsky, who was nominated for an Oscar in 2009 for The Final Inch and won an Emmy in 2004 for Heart of the Country. BroBible reports that Brodsky got exclusive access to the families for over 18 months to create Beware the Slenderman.

Check out HBO’s promo poster for the movie:

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The official synopsis via BroBible:

“Shot over 18 months with heartbreaking access to the families of the would-be murderers, the film plunges deep down the rabbit hole of their crime, a Boogeyman and our society’s most impressionable consumers of media. The entrance to the internet can quickly lead us to its dark basement, within just a matter of clicks. How much do we hold children responsible for what they find there?”

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