Dressed in a black suit and with no visible face, the Slender Man is an apparition with arms that stretch wide to entrap his chosen prey.
Due to his attire, the Slender Man has been linked to the notorious "Men in Black", who are thought to be government agents who harass and threaten UFO witnesses, and who some believe to be aliens themselves who take human form.
The only problem is that the Slender Man is actually a fictional character.
The Slender Man was born within the 'Something Awful' forums in 2009, when a thread was created to challenge members to create fake photo-shopped paranormal photographs. The idea, however, was not just to create these images, but to also filter them through the Internet, in a bid to convince those who sought out such things that the images created were "real" ghost/alien photographs.
Despite the Slender Man being a complete fabrication from the beginning, his legend has spread like wild-fire throughout the Internet as a successful "Internet MEME", a phrase used to describe a catchphrase or concept that spreads quickly from person to person via the Internet, much like an esoteric inside joke, that evolves with time, much like other famous memes such as the Lolcats and emoticons.
A YouTube sensation of sorts, the 'Marble Hornets Project', is a series of video clips that were supposedly recorded by a teenager named 'Alex'. The segments of video taken from Alex's camcorder tapes follow him as he becomes increasingly concerned by the fact that something sinister is apparently stalking him.
Whilst Alex himself has mysteriously vanished, the video clips he left behind him become increasingly disturbing, as the Slender Man peers from distant corners. While the Slender Man itself is completely fictional, the 'Marble Hornets Project' and other Internet adaptations of the entity have greatly increased the range of the Slender Man myth's reach, with some unfamiliar with the legend's origin taking it at face-value, and they in turn spread the legend's grip further still.
Like its origins, the appearance of the Slender Man remains subjective, varying across related media. It is typically depicted as being an abnormally tall (ranging anywhere from 7-foot-tall (2.1 m) to the size of a large tree or house), skinny, humanoid figure, with a faceless or deformed head, usually lacking several or all of the face's sensory organs. It is usually depicted as wearing a a plain black suit and tie, reminiscent of the Men in Black, though this is often implied to be a part of the entity's body or a form of shapeshifting. It is occasionally shown to have something of a malleable body, enabling it to stretch and contort its limbs and body in to a variety of shapes, as well as produce a number of tentacle-like appendages, which are often shown has having the ability to ensnare the Slender Man's victims.
The Slender Man is typically depicted as an observant, malevolent, and lurking being, stalking its victims for long periods of time while slowly driving them insanity with paranoia and sickness. Other adaptations point to a more violent Slender Man, which kills its victims by eviscerating them and hangs plastic bags of blood from trees.
Besides the ability to physically grow extra limbs, the Slender Man has a number of psychologically-based powers, typically the ability to induce memory loss, insomnia and insanity in its victims. In some cases, the extent of its power over the mind has seen it creating mindless minions seemingly bent to his will. The Slender Man's physical presence has been shown to induce coughing and sickness, as well as various degrees of audio-visual distortion in electronic media, a trait often used to denote its presence in Slender Man video series. In extreme cases, it has also been depicted with the ability to warp time and space and teleport.