Three 12 year old girls (Anissa Weier, Morgan Geyser, and Payton Leutner) have a sleepover and then go for a walk in the woods the next day. During a game of Hide and Seek, Anissa and Morgan stab Payton 19 times and leave her for dead, yet promising to get help.
They never do.
This, all in the name of Slenderman. They were caught 5 hours later, walking to find Slenderman's mansion in the woods.