The Slenderman myth is explored in a new documentary called Beware the Slenderman, premiering tonight on HBO.

Those familiar with internet memes from the late 2000s will surely recognize the subject of HBO's latest documentary. Slenderman, an unusually tall fictional creature, became popular in 2009. The character is frequently depicted stalking children and superimposed into historical photos.

Beware the Slenderman follows how the myth influenced two 12-year-old girls to commit an unspeakable act in 2014.

As HBO describes, Beware the Slenderman "tells the true story of 12-year-olds Morgan Geyser and Anissa Weier, who lured their best friend into the woods, stabbed her 19 times and confessed they did it to appease a tall and faceless man known online as Slenderman. Featuring heart-wrenching access to the girls' families, the haunting documentary plunges deep down the rabbit hole of their actions, exploring how the dark corners of the Internet can influence society's most impressionable young consumers of media."

The documentary features the expertise of psychologists like Dr. Michael Caldwell, Psy.D, who explains Anissa's delusional disorder, which is the inability to distinguish between what is and isn't real. "Adults may have simply eccentric religious or spiritual beliefs or conspiratorial beliefs or things like that, and live out their life with no problems at all," Dr. Caldwell explained.

Beware the Slenderman was directed by Irene Taylor Brodsky, whose documentary short The Final Inch received an Oscar nomination in 2010.

Beware the Slenderman premieres on HBO at 10 p.m. ET/PT tonight, and will be available at the same time on HBO Go and HBO NOW. The film is expected to be available for streaming after the premiere, for those who aren't able to watch it live.

To live tweet and follow along with others watching the movie premiere of Beware the Slenderman, use hashtag #BewaretheSlenderman on Twitter.

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