From Akashic Records
Slenderman is a cryptid created in mid-June 2009 by several forum-goers from the Something Awful forums.
The typical appearance of Slenderman was originally that of an impossibly tall man in a suit with a distorted, blank face. The fear it inspired came from both the odd proportions of the body and the distortion of its face, which encouraged the viewer to stare deeper into it. Later posters would give him tentacles and attribute various creepypasta to him. Both tentacled and non-tentacled versions are accepted in Slenderman canon.
Before the creepypasta was widespread, Slenderman was used as a sort of "where's Waldo", with users hiding him in photos. As his popularity grew, creepypasta would become attributed to him. The most generally accepted of the stories would be ones that had a subtle level of realism that readers could suspend their disbelief to, while outright storytelling with the military and scientists are frowned upon as cheap science fiction.
Most recently, two projects have developed adopting the Slenderman mythos into fiction: Marble Hornets, a series of online videos, and Just Another Fool, an online blog. Just Another Fool ended in January 2010, but Marble Hornets is currently ongoing.
Create Paranormal Images - The Something Awful Forums - The thread that started it all.
Just Another Fool