The Slenderman myth originated from the Something Awful forums, and was one of the many pictures submitted to a paranormal pictures contest. The user who had submitted the intresting photos had created the story of Slenderman, a strange creature that stalked children.
The Slenderman is a strange being, seemingly capable of causing a strange sickness, mass paranoia/fear in his victims, and even teleporting.
The Slenderman is commonly depicted as a tall, thin being wearing a suit, with no face, and sometimes tentacles. He is also known though the Operator Symbol, which signifies his no face feature.
Also, the Slenderman is also known for causing severe memory loss, and controlling people/driving them to madness. In some cases, chasing them to no end.