Slender Man is a fictional supernatural character.  He is depicted as thin, unnaturally tall, with long, tentacle-like arms, a featureless head and face, and he wears a black suit.  It's just an internet meme that pops up in creepypasta stories.  In this story, there are three young girls in the sixth grade, Morgan Geyser, Anissa Weir, and Payton Leutner.  One of the kids believed so strongly that Slender Man is real, and she came up with a plan to prove it.

Drinking The Cool Aid

RESOURCES:

https://www.cnn.com/2019/10/25/us/slenderman-victim-speaks-trnd/index.html

https://www.foxnews.com/us/wisconsin-girl-15-sentenced-in-slender-man-stabbing-case

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

https://nypost.com/2019/10/24/slender-man-stabbing-victim-payton-leutner-on-the-friends-who-tried-to-kill-her/

https://www.chicagotribune.com/midwest/ct-wisconsin-slender-man-payton-leutner-20191024-odiauulr4jgwxpysazdmah2qre-story.html