Beware the Slenderman, a 2016 biographical crime documentary film about two girls who murdered one of their friends in order to appease "The Slenderman".
"The Slenderman" is a fictional faceless monster reportedly featured in a horror story on the website fanfiction.net. It was published online in 2009 and quickly gained an urban myth status.
The creature was completely fictitious, though two 12-year-old girls from Wisconsin apparently believed the story. In 2014 they lured a friend into the woods and stabbed her to death; while being interrogated for the murder they both explained to detectives that they were compelled to kill their friend in order to become proxies of Slenderman.
This documentary by Irene Taylor Brodsky explains the story of what happened and how the bizarre case unfolded.