According to The Washington Post, the Slender Man is a character that was invented in a forum in 2009. The forum users Photoshopped images of a gaunt figure with skeletal limbs into old pictures and invented back stories for their creations.
Slender Man is a fictional character created by Eric Knudsen on the Something Awful Internet forum. The purpose of the forum thread was to edit images of supernatural entities into photographs for a photo contest. Knudsen contributed two black and white images that displayed groups of children. He added an image of a tall, thin spectral figure wearing a black suit to both photos. Then he supplemented his submissions with fake eyewitness accounts. In the accounts, the witnesses told stories of how Slender Man abducted the groups of children. Subsequent posters to the forum expounded upon the story of the child abductions while adding more details about the fictitious character Knudsen had created.
In an interview conducted by Know Your Meme, Knudsen stated that the creation of the Slender Man was influenced by H.P. Lovecraft, the short stories of Stephen King, the surreal imaginings of William S. Burroughs, and the video games "Silent Hill" and "Resident Evil." He also cited influences of a number of other stories and myths to create a character that would cause unease and terror in the general public.