In its inception, Slenderman was created as part of a scary pictures photoshop thread on Something Awful, what started as a simple tree-themed boogieman has become what many consider to be one of the most terrifying monsters in fiction. After several "found footage" style video series on YouTube and countless blogs, the myth of the Slenderman has grown. In short, Slenderman is an entity that appears in the form of an abnormally tall man, around 8 to 10 feet tall wearing a suit, but without any facial features or hair. Early entries in the mythos depicted its lack of face instead as a ghoulish face that would appear different to each person who saw it. It was said to be capable of causing those who witness it to be drawn to it, almost hypnotically. This can be seen still in various ways throughout the various series: Literally in those who are drawn to the Slenderman when in its presence, those who become slaves to the creature or, more abstract, those protagonists who hunt after it in the hopes of knowing why it's stalking them.
In many cases, the Slenderman is known to induce various forms of illness in those who encounter it: physical illness displaying itself in the forms of severe coughing, nausea, nose bleeds, or mental stresses ranging from increased paranoia (though, perhaps, not without merit) to outright inducement of madness.
Another disturbing property of the Slenderman is that it is capable of changing the length of its appendages, warping its already unusual appearance into something even more inhuman than before. It is capable of creating tendrils from its back, it is said, to ensnare victims or even simple locomotion.
But its body is not the only thing it can warp. It is believed that it has the ability to warp time and space, at least locally, however it may desire to deal with those whom it has targeted. Such unlucky souls have found themselves exploring houses in the middle of the day, only to be sent from room to repeating room for a few minutes, only to discover once their bizarre trip through space is concluded that bright daylight has suddenly become night. Others dealing with the being have even found themselves transported great distances from their homes to familiar (if they're lucky) locations, with no explanation of how they got there.
It is believed that the Slenderman has connections to the elements of fire and water. Associations of the Slenderman to the Great Floods of myth are not unusual, and it is possible that water is indicative of its presence. And fire is believed to be the Slenderman's tool for clearing the slate, as it were; fire has played a role in much of the Slenderman mythos, from the burned out husks of buildings where our heroes tread to the fires that nearly cost them their lives.
Perhaps most frightening of all, it is never certain just what the Slenderman does with those whom it abducts. Some victims are believed to have become servants to the being. Others, perhaps just bait for further victims. As for the others, perhaps it is for the best that their grisly fates are left unspoken. But for many it may never be known for sure…
As of current, there are several different video series that fans have flocked too.
The first, Marble Hornets, is considered by many to be the best of the lot. It was, next to Victor Surge's initial postings on Something Awful, the first major series to break into the public consciousness, even garnering the attention of movie critic Roger Ebert
. Other fan favorites include EverymanHYBRID and the Marble Hornets-esque Tribe Twelve. Of additional recent interest are the series DarkHarvest00 and The Angel's Game.
There are, additionally, countless text blogs across the net, each telling their own Slenderman Tale. Unfortunately, the quality of these stories can be rather hit or miss.
What follows are Infodumps for each of the major "Vlogs"
EDIT: Corrected grammar