Years on, mother of the Slender Man stabbing victim shares new details of her trauma.

Sixteen-year-old Anissa Weier today learned she will spend the rest of her young adult life confined to the walls of a mental health facility.

The Wisconsin girl was handed the maximum 25-year sentence by a Waukesha County Circuit Judge over the attempted murder of her classmate in 2014; a crime, Weier said, that was an attempt to appease a fictional horror character known as The Slender Man.

Compelled by online stories about this tall, faceless boogeyman, Weier and her friend Morgan Geyser, then aged 12, lured Payton Leutner into a wooded park near the city of Milwaukee on the morning of May 31, 2014. There Geyser stabbed Leutner 19 times with a kitchen knife as Weier egged her on: “Go ballistic, go crazy,” she urged her friend.

It was a necessary sacrifice, the pair believed. One that would prove their loyalty to The Slender Man, and protect them and their families from harm. After the ‘murder’ was carried out, they believed they would become his servants or “proxies” and be allowed to live in his mansion, which in their minds, was located 300km north in the Nicolet National Forest.

After washing the blood from their hands in a Walmart bathroom, they set off toward it on foot, singing and crying. They were arrested two hours later.

Anissa Weier pleaded guilty in August to being a party to attempted second-degree intentional homicide, but claimed mental illness meant she wasn’t responsible for her actions. In September, a jury agreed.

By the time she is released, Weier will be 37.

Thursday's sentence was welcomed by the Leutner family, who have remained relatively quiet about the case. It wasn't until Payton's mother, Stacie Leutner, issued a victim impact statement to the court ahead of Weier's sentencing that the world learned the ongoing psychological trauma caused by the attack.

In the letter, the woman wrote that for months after returning from hospital Payton spoke only to her and was unable to sleep alone.

"When she did finally move back into her bedroom she refused to unlock the windows or open the curtains. She slept with scissors under her pillow," the statement read. "She couldn’t sleep unless I was in the room next to her and could respond to three knocks on her wall with three knocks on mine signifying 'I love you, you’re safe, and I’m here'."

Of course, the physical damage was significant, too.

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The knife pierced the young girl's arms, legs, liver, stomach and pancreas, and missed a major artery near her heart by just the width of a human hair.

The near-six-hour surgery to save her left her with even more scars, two of which stretch from below her neck to below her belly button, and are still "red and angry" three years on. Every day they tingle and ache, reminding her of their presence and of the attack.

These marks have made ordinary events difficult for the teen.

"Shopping for homecoming dresses leaves only a few options because far too many dresses will show off her scars," Mrs Leutner wrote. "Beach vacations are harsh reminders that swimsuits aren’t made for young girls with 25 scars."

Payton's younger brother, too, has been impacted by the crime - three years on, he still "struggles with anxiety, feelings of abandonment", according to Mrs Luetner. As was her parents' marriage, which dissolved amid the court hearings, media requests, doctor and therapy appointments.

"We are all still trying to figure out what it means to live in the 'new normal' that we have been forced to endure. We didn’t choose this life, it was thrust upon us unwillingly and we have had no choice but to stand up and deal with it in the best way we knew how," the mother of two wrote.

Before being sentenced, Anissa Weier offered a brief apology for the attack and the trauma it caused.

"I do hold myself accountable for this and that I will do whatever I have to do to make sure I don't get any sort of delusion or whatever again," Weier told the judge, according to Associated Press. "I want everybody involved to know I deeply regret everything that happened that day. I know that nothing I say is going to make this right and nothing I say is going to fix what I broke."

Morgan Geyser is due to be sentenced in February, having pleaded guilty to attempted first-degree intentional homicide in September. The plea deal she struck with prosecutors means she will avoid a prison sentence, and - like Weier - will be confined to a mental health facility.

Prosecutors have asked that she be held there for 40 years.

Payton Leutner, meanwhile, will live with the scars of May 31, 2014, for the rest of her life.

As her mother wrote to the court, "the emotional trauma she has endured will remain with her for a very long time. It will fade and feel less intense as the years progress but it will always be there, menacing in the back of her mind reminding her not to trust too deeply or love unconditionally, because the last time she did she nearly died."

For more on this case, read "TRUE CRIME: The 12-year-old girls who stabbed their friend because ‘Slender Man’ said to."