One late spring day in 2014, three girls entered the woods in Waukesha, Wisconsin. Two walked out unharmed. A 911 call made not long after revealed the hazy outline of a vicious attack—one of the girls had been found by the side of the road covered in blood, having crawled there to get help. In the days and weeks that followed, details emerged that were no less disturbing: The three girls, all 12 years old, were best friends. The victim had been stabbed 19 times with a 5-inch blade and had barely survived. After being taken into police custody, the other two girls told interrogators what had happened: They had lured their friend into the woods to kill her so that they could appease someone called Slenderman.