The Slenderman is a boogeyman born from the Something Awful forums that manifested in the real world in 2014 when two pre-teen girls stabbed their friend 19 times to please The Slenderman. "A Self-Induced Hallucination" is director Dan Schoenbrun's documentary about The Slenderman that he made entirely from archival footage.
"The Slender Man. He exists because you thought of him. Now try and not think of him."
-Username "I," posted June 15th, 2009 on the Something Awful forums. (User was later banned for "post(ing) like a weird fucker.")
"Why I Spent Months Making An Archival Documentary about The Slenderman" (Filmmaker Magazine)