User blog comment:HuntressNight22/All About The Slendy Talk!/@comment-24569600-20140428043848/@comment-24569600-20140428045931

Actually he was made for a contest, by a guy named Victor Surge on the Something Awful forums back in 2009. He isn't tied to any particular story.

Source: http://en.wikipedia.org/wiki/Slender_Man