A teenager who allowed a girl to be viciously stabbed 19 times in a bid to please a horror character has avoided a jail term.

Anissa Weier egged on friend Morgan Geyser as she attacked Payton Leutner so the pair could avoid the wrath of supernatural character Slenderman.

All three girls were 12 years old when the attack took place in a Wisconsin woods in May 2014.

Leutner managed to crawl out of the park before being spotted by a passing cyclist.

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She survived her wounds.

A jury have found that Weier – who admitted her participation – was mentally ill at the time, meaning she will spend the next three years in hospital rather than jail.

Jurors were told the victim was found by a passing cyclist. AP

Weier, now 15, pleaded guilty in August to being a party to attempted second-degree intentional homicide.

The jury’s job was to find whether she was ill at the time.

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Three psychologists testified that she suffered a shared delusion that she and Geyser had to kill Leutner to become Slenderman’s servants and prevent the character from killing their families.

Jurors saw clips of Weier’s police interrogation, during which she appeared to move from belief in Slenderman to doubt to a realisation that he wasn’t real.

Prosecutors tried convincing the court that Weier went along with the attack to preserve her friendship with Geyser.

Geyser has pleaded not guilty by reason of mental disease to being a party to first-degree attempted intentional homicide.

Her trial is set to start in October.