While most internet memes are harmless, there’s one in particular that resulted in the inspiration for a brutal attack by two Wisconsin girls who stabbed their friend 19 times.

A new documentary titled “Beware The Slenderman” explored the true story of how fictional internet character Slenderman motivated two 12-year-old girls from Waukesha to attempt murder in 2014, HBO announced Wednesday. In the wake of the upcoming film slated to debut Monday, here are five facts about Slenderman and the Wisconsin stabbing:

Slenderman or Slender Man — a thin, tall figure wearing a black suit and sporting a blank face — sprouted on an internet forum in 2009 as a fictional supernatural character. User Victor Surge created a Photoshopped image on a forum inviting people to make paranormal pictures through photo editing. The thread was part of a website called Something Awful – an online comedy platform where users can post various content.

The original photo, which featured the thin figure behind a crowd of children, was accompanied by a brief description: “We didn’t want to go, we didn’t want to kill them, but its persistent silence and outstretched arms horrified and comforted us at the same time… 1983, photographer unknown, presumed dead.”

The image caught wind on the internet and eventually became more mainstream, appearing on forums like 4chan and inspiring a web series called "Marble Hornets." The character also appeared on Creepypasta, a website featuring horror stories, which was reportedly the first time that the Wisconsin girls came across the Slenderman story.

The two 12-year-old girls allegedly lured their friend into the woods in Waukesha and stabbed her 19 times in an attempt to please the Slenderman. The victim, 12-year-old Payton Leutner, was found near the woods barely alive the day after the three girls planned to have a sleepover. The attackers, Morgan Geyser and Anissa Weier, were trekking along the highway when authorities caught up to them. They eventually confessed to the attack, citing Slenderman as their main motive.

Evidence revealed that the attack was meditated and very violent. Geyser and Weier decided after discovering Slenderman's story online that they needed to prove their loyalty to him and his existence. They took on the role of being a “proxy," which required them to kill someone. In order for this to happen, the girls chose Leutner as their victim. They attacked her in the woods on a Saturday morning, stabbing her 19 times in the pancreas, stomach, liver and almost hitting an artery close to her heart.

A Wisconsin state appeals court ruled that Geyser and Weier would be tried as adults. Both girls pleaded not guilty to the attack by reason of mental illness and could face up to 65 years in prison if found guilty. Weier’s parents said that their daughter should not be tried as an adult because Wisconsin’s “outdated” laws are not up to speed with juvenile brain development research. A trial date has not been set for either girl.