15-year-old Morgan Geyser has been sentenced to 40 Years in a mental institution for stabbing a former classmate in 2014, with the hopes winning approval from the fictional "Slenderman." Geyser pleaded guilty in October to a first-degree attempted murder charge, which kept her from facing jail time.

Lawyers and doctors have said that when Morgan and her friend, Anissa Weier, committed the crime, Geyser was schizophrenic and was diagnosed with a psychotic spectrum disorder. Weier also pleaded guilty to a lesser charge and is serving 25 years in a psychiatric hospital. According to CNN, Geyser was previously diagnosed with schizophrenia prior to the attack.

Geyser's lawyers were arguing for a shorter sentence but a doctor who testified for the prosecution said she was still a threat to the public and claimed she said she was hearing voices.

Judge Michael Bohren said this was a reasonable and fair sentence and that her lawyers can petition for her release every six months. Bohren says he doesn't want Geyser to end up in the community and then fall back into a similar situation and end up back in the courtroom.

Who is Slenderman?

Slenderman is a nightmarish character born on the internet, whose influence has stretched into the real world. What was once fiction turned into a major urban legend and eventually the inspiration for this horrific crime. Slenderman is often portrayed as a tall man with a white head wearing a suit. He is often seen with tentacles that make him look an evil spider.

He is believed to be able to attack with mind control and is able to appear and disappear anytime he chooses.

The creator of Slenderman is a Creepy Pasta user named Victor Surge. The surge started a competition, where he would take photos from the past and create the most-terrifying monster and put them in the photos. This turned Slenderman into an overnight creepy sensation. The success of this character has led to computer games, books, and animated videos.

Mothers of defendants speak out

Both mothers of the defendants spoke to ABC News on "20/20" and said they could not have imagined that their kids were capable of hurting someone. They both said they did not see any warning signs from their kids that they were going to hurt someone else. Both the defendants and victim Payton were friends and would often spend time with each other at their homes.

Morgan's mom Angie said she was aware of her interest in Slender Man and even talked about him with her. Angie said that the stuff they read was dark material but that she was reading Stephen King books at her age, so she wasn't worried.