The Slender Man is a fictional (Or is he) Creepypasta and one of the most famous of his kind.
History[]
When The Mind Flayers took over the Universe, they time travelled back to the dawn of time to make a killer for them. They took the body of Burke Slender, their greatest nemesis, and transformed him into an abberation intent on killing everyone. Slender Man did this for eons, before arriving on earth c. 5000 B.C. He became fascinated with the human culture and studied them for years. Eventually, when the Red Death came to earth he decided to go back to killing them, but instead of killing them all at once, he decided to stalk and hunt them one by one for eternity. At some point he captured a member of the Q continuum who in exchange for freedom gave Slender Man a mansion he could teleport anywhere.