Marble Hornets is an alternate reality game and YouTube horror series involving the Slender Man character. As of 2014, there have been three seasons of Marble Hornets, each telling the unfolding tale through YouTube videos interspersed with interactive phases where the audience uses clues to discover more information and move the storyline forward. Slender Man is one of the primary antagonists of the series.
The Slender Man character was originally created in 2009 by Eric Knudsen on the Something Awful forums as part of a Photoshop contest to create creepy images. Knudsen Photoshopped a tall, suit-wearing figure with a blank face into pictures of children, building a fictional narrative around the images. Other writers took up the character and narrative, building a shared-world mythos around the character. The first video series based on Slender Man was Marble Hornets, begun later that same year.
In 2013, it was announced that Marble Hornets would become a feature film. In addition to this project, Slender Man has appeared in multiple video games and independent films. The popular game Minecraft even features a race of monsters based on Slender Man, called Endermen, who are a powerful threat to players and play a role in Minecraft's endgame.