Children believe in things like fairies who collect teeth and rabbits who hide eggs, so it makes sense that by junior high they could move on to urban legends about impossibly tall men lurking in the woods and sprouting tentacles. It’s not surprising that a child would develop an interest in the internet nightmare machine known as Slenderman; what is surprising is that such an interest would end up drawing innocent blood. This fact alone is enough to make the new HBO documentary Beware the Slenderman a chilling experience.

But it doesn’t stop there.

It would be easy to skim a news story about Morgan Geyser and Anissa Weier—two 12-year-old girls who lured their best friend into the woods and attacked her with a knife in the spring of 2014—and dismiss their claim that their crime was committed at the behest of Slenderman as a panicked excuse. It would be very easy to blame what they did on the child-corrupting power of the internet, or unchecked mental illness, or childish naievete turned brutal. The real truth is a powerful, singular cocktail of all of this and more. This complex understanding of the crime, coupled with director Irene Taylor Brodsky’s storytelling power, has the effect of conjuring the very real sense of a monster just over your shoulder.

Whether he’s viewed as a literal creature by Geyser and Weier or as an internet folklore phenomenon by several experts interviewed for the film, Slenderman is a real presence as far as this documentary is concerned. During police questioning, Weier and Geyser rattle off details of his powers, appearance and physical attributes like they’re reciting from a zoological textbook. When it comes time for experts to weigh in, Slenderman is not dismissed; he’s compared to centuries-old figures like the Pied Piper of Hamelin and his child-bewitching music. His captivating appearance, and cyber-reams of stories tied to classic boogeyman mystique, make him irrestible to scholars, storytellers and schoolchildren alike, and the film never stops reminding you just how widespread that aura really is.

Brodsky then pushes the aura further with numerous visual examples of Slenderman, from cutesy fan art to convincing found-footage horror shorts. At first, this seems like a simple device aimed at proving the worldwide reach of the character, but as the film progresses and more examples emerge (including Geyser’s own unsettling drawings), it creates a blur over reality. The leap of reasoning a 12-year-old girl must make to believe in this creature suddenly becomes a very short leap for the viewer as well, particularly as more and more details of the buildup to and aftermath of the crime itself are revealed.

Which brings us to the final layer of dread that makes Beware the Slenderman so unnerving: Weier and Geyser’s parallel stories. Both are social outliers who plumb the depths of the internet for a place to belong, both seem to carry out their crime with an eye toward its necessity rather than merely seeing what will happen if they do, and both reveal particular vulnerabilities as the film progresses and their respective trials draw near. What chills you to your marrow, when Geyser says “It was necessary” as though she just stepped on a bug, will melt you when you see how loved these girls are and how capable of love they are in turn. In spite of their monstrous deed, these girls are very, very human.

What, then, drove them to this? How is a monster from the internet capable of casting such a spell? How is such a reckless act so permeated with time-tested, even ancient mythic connections and premeditations? The answer is not simple, and I’m not even entirey sure the film itself can weave together a specific answer. The questions Beware the Slenderman asks of its subjects, though, will knock around the darkest parts of your brain until they practically live behind your eyelids. The film’s blend of true-crime drama and cryptozoological horror provides an easy, early hook, but its more complicated conclusions about scary stories, mental health and childhood violence are what don’t let go. In the end, this film and its eerie blend of fiction and fact will convince you monsters are real, even if they’re the ones in your own head.

Beware the Slenderman premieres tonight at 10 p.m. EST on HBO.