He is tall, he is slender, he is dark, and, for two teenage girls in Wisconsin, he was an inspiration for evil. He is Slender Man.

Sources:

https://www.nytimes.com/2014/06/08/us/milwaukee-suburb-tries-to-cope-with-slender-man-stabbing.html

https://www.nytimes.com/2018/08/15/movies/slender-man-timeline.html

A documentary film on the incident called Beware the Slenderman, directed by Irene Taylor Brodsky, HBO Films 2016

https://en.wikipedia.org/wiki/Slender_Man