PterodactylTamer
In that moment, before what little remained of his sanity was completely torn apart, he heard three words whispered in his ear.
The slender man.
The Slender Man is a being, initially human like, who's never been seen up close before. He takes children, and they become the 'Lost One's', or the 'Slender Children.'
Abby makes a deal with the Slender Man. They will play a game. If she wins, her and her sister will walk away free. If the Slender Man wins, Abby and her sister, Mary, will become Slender Children.