On May 31, 2014, two 12-year-old girls from Waukesha, Wisconsin allegedly lured their classmate into the woods and stabbed her 19 times to impress the Slender Man - a fictional character created for a 2009 Photoshop contest. He is depicted as a tall, faceless man in black suit with tentacles growing out of his back, and hiding in forests or stalking children.

Slender Man is now the subject of an HBO documentary titled "Beware the Slenderman" which aired Monday, Rolling Stone reported. The documentary shows police interrogation hours after the arrest of Morgan Geyser and Anissa Weier. This digs into the psychological aspects as to why the girls tried to stab their friend, Payton "Bella" Leuster.

"Beware the Slenderman" is directed by Irene Taylor Brodsky, and it examines the rise of the Slenderman myth online, via online horror stories and art known as "creepypastas," message boards, fan sites, and social media. The fictional character has amassed a huge online fan base, and this is where Geyser and Weier's imagination has been captured.

The two girls, who were armed with a knife from Geyser's kitchen, aimed to kill Leutner in order to please Slenderman and protect their families from falling victim to his wicked acts. Both girls sincerely believed in Slenderman. Weier said it was Geyser's suggestion to kill Leutner so they could become proxies of Slenderman and live in Slender Mansion.

Both Geyser and Weier have been incarcerated for the last two years. Trial date is still pending, but they are set to be tried as adults for ttempted first-degree intentional homicide. If the girls are convicted, they could be sentence to up to 65 years in state prison.

According to The Guardian, "Beware the Slenderman" is not like any true-crime documentaries because the focus in not on who but why and how. The film shows untouched and un-doctored evidence, which makes it chilling. Geyser and Weier discussed in detail as to how they plan to carry on the murder. "Beware the Slenderman" premieres at 10pm on Monday 23 January and airs in the UK on Sky Atlantic on Thursday 26 January at 10pm.