Scary horror game SANATORIUM based on SLENDER

[Tiger]

I don't think it's copyrighted. As it began as an "urban myth" (some dudes PSing old photos and throwing them out there claiming it to be true photographs). People have been using the Slenderman mythos a lot in different contexts, such as marble hornets, Slender game and books (believe it or not), it's kinda like santa.

Well, The slenderman has been a myth for hundreds of years, so it's not really just "some dudes PSing old photos".