Wait, who is the Slender Man again?
The Slender Man -- sometimes spelled as one word and occasionally referred to as "Slender" -- is a towering, faceless humanoid who dresses in a suit and stalks lonely children. Entirely fictional, he was created in 2009, when a user of the internet forum Something Awful submitted a doctored photo for a paranormal image contest. The character quickly exploded in popularity and became a fixture of horror sites (namely Creepypasta), inspiring countless videos and pieces of fan fiction.
OK, so what happened?
In May 2014, Morgan Geyser invited Anissa Weier and Payton Leutner to sleep at her house to celebrate her 12th birthday. The next morning, Morgan and Anissa took Payton to a nearby wooded area in the Waukesha suburb of Milwaukee and stabbed her 19 times with a kitchen knife, leaving her to die.