The Slender Man is a fictional character that originated in an online art contest in 2009, before becoming a popular meme.
It is what drove Anissa Weier and Morgan Geyser to stab their friend Payton Leutner 19 times in May 2014 in Wisconsin.
The girls feared their own families would die if they did not follow its orders.
The mythical creature is often depicted as an unnaturally tall, thin figure with a blank, featureless face, wearing a black suit.
The character is said to have creepily long, tentacle-like arms, which can be extended to capture prey.
It is described as stalking, abducting and traumatizing children, and – depending on variations of the urban legend – can cause memory loss, insomnia and paranoia.
It is also said to be able to create distortions in photographs and teleport.
The character is understood to have originated in a Photoshop contest on the forums of comedy website Something Awful in 2009.
It then went viral, with numerous works of fan art and short scary stories – known as ‘creepypasta’ – published online.
The character has developed its own life online, with two feature-length films funded in part by Kickstarter appearing in 2012, along with many amateur video games and pop-culture references.