MILWAUKEE » A pair of 13-year-old Wisconsin girls accused of luring their friend into the woods to stab her as a sacrifice to the fictitious horror character Slender Man are set to formally enter their pleas in the case.

The teens from the Milwaukee suburb of Waukesha are being tried as adults and face decades in prison in connection with the 2014 attack. They are expected to appear in court at a brief arraignment hearing Friday afternoon.

Investigators have said the two plotted for months to lure their friend Payton Leutner into the woods after a sleepover. They intended to kill her, police say, to win the favor of Slender Man, described as an unnaturally tall and thin man who wears a dark suit and has no visible facial features. Slender Man stories have proliferated online in recent years.

They wanted to kill for him, in part, to prove his existence, police documents allege.

The girls, 12 at the time of the stabbing, believed they would have a home in Slender Man’s mansion if they carried out the attack, police say.

After stabbing their friend 19 times and leaving her for dead, they started walking to a forest 300 miles away, where they believed he lived, according to police documents. Payton survived.

Defense attorneys, who couldn’t be reached for comment Thursday, have argued previously that the case belongs in juvenile court, saying the adolescents suffer from mental illness and won’t get the treatment they need in the adult prison system.

But Waukesha County Circuit Judge Michael Bohren decided this month that the girls should be tried in adult court despite their age, saying if they were found guilty in the juvenile system they would be released at 18 with no supervision or mental health treatment. Keeping them in the adult system would protect them longer, he said.

The Associated Press has not identified the girls because an appeals court could move their cases to juvenile court, where proceedings are closed.

The girls face 65 years in prison if convicted as adults. They have been in custody since being arrested the day of the attack.

Associated Press writer Todd Richmond contributed to this report.