Beware the Slenderman: Watch the Trailer for HBO's Bone-Chilling Documentary

While Slenderman is fictional (we hope), he did inspire a shocking attack in Wisconsin. In 2014, Morgan Geyser and Anissa Weier, both 12 years old at the time, lured their best friend into the woods and stabbed her 19 times; they claimed they did it for Slenderman. The documentary follows the families of the attackers for over 18 months, delving into how and why the girls did what they did.