Greetings from SomethingAwful! I made an account just for this reason.
The Slender Man originated by a great man named Victor Surge, somewhere between pages 4 and 6 in this thread.
Marble Hornets is a sort of ARG made by ANOTHER member of SA, just to scare us and eventually lead up to a (hopefully) great end.
Well, I hope that this post cleared some stuff up, and ... yeah.