This film was originally reviewed at the 2016 Fantasia International Film Festival and will debut on HBO on January 23, 2017.

On May 31, 2014, three 12-year-old girls ventured into a forest in Waukesha, Wisconsin. Two of the girls then stabbed the third 19 times and left her for dead. They were quickly arrested, and showing little in the way of remorse, told police that they had committed the crime because of the Slenderman.

The Slenderman is seen as a modern-day Pied Piper.

This deeply troubling documentary endeavors to tell two tales, detailing the events surrounding that attempted murder (the victim miraculously survived), and the story of the mysterious man in question.An entirely fictional creation, the character first appeared online in June 2009, a tall, faceless entity loitering in the background of pictures that were entered into a Photoshop contest on the Something Awful forum.Since then Slenderman has becomes something of an Internet sensation, appearing in art, games, photos and fan fiction, and spawning countless memes and viral videos.In some versions he has giant tendrils, in others he can teleport and read minds. And more-often-than-not he’s evil, leading children away like a modern-day Pied Piper.For the confused, impressionable girls in question, he was all-too-real, haunting their nightmares, and apparently urging them to kill their friend.In chilling footage of their police interrogation, they claim to be the Slenderman’s proxies, and attempted murder “because it was necessary.”Director Irene Taylor Brodsky explores their home life in an effort to figure out how such darkness entered their world. And on the surface it seems normal – baby pictures and videos feature two happy, seemingly carefree children.But interviews with their parents highlight issues that both girls developed, linked to loneliness, anger, and a distinct lack of empathy. One of the fathers has mental health issues, and there’s a suggestion that his daughter has inherited the same disorder.Experts interviewed over Skype offer their own reasons, with a psychologist referring to their “delusional disorder” and famed evolutionary biologist Richard Dawkins calling the influence of the Slenderman a "virus of the mind."But while the documentary asks lots of tough questions, there are few answers, the filmmakers instead laying out the evidence and leaving the audience to decide. Which is fine for a story this complex, were it not for the fact that the film also feels somewhat incomplete.That’s because there’s a huge focus on the trial, where we hear yet more details of the crime and debate over whether or not the children should be tried as adults. But those legal proceedings are still ongoing, so when you’re expecting some kind of resolution on that front, the film just ends.We also spend a good deal of time getting to know the families of the perpetrators as they struggle to come to terms with what their children have done, but there’s no word from the victim or her parents. They may well have declined to participate in a film about the heinous act that nearly killed their daughter, but in failing to tell both sides, it does mean there’s something of an imbalance to the storytelling. Beware the Slenderman is much better when dealing with the nature of the modern myth; how urban legends can quickly spread across all forms of media, and how they can effect young, impressionable minds.For that reason it’s a powerful cautionary tale of the relationship children have with the Internet, and how what is fun for some can turn into a dangerous obsession for others.And in a bizarre twist worthy of a story this strange, the girls themselves have now gone viral, featuring in a slew of pictures and videos alongside the Slenderman, meaning that they have now become a part of the legend.