“Slender Man is scary not because of what you know about him but because of what you don’t know. … The character is a blank canvas for our fears but also for online storytelling. Now the namesake of a new horror film, Slender Man started to take shape in an online forum nearly a decade ago, at a time when daily life was shifting to social media and the border between the online world and the real one was starting to blur. Here’s a look at Slender Man’s evolution.”