Sure thing. I didn't want to post spoilers for those whom haven't seen it yet so be warned that the following gives away the whole movie.
Session 9 is only a supernatural film due to the very last line spoken in the film which was from the end of the ninth and final session recording of an asylum inmate.
The story opens with a renovation crew coming to fix up an asylum closed down in the 80's. As most asylums it is deemed a historical landmark and therefor is protected from being totally knocked down.
The guard watching the property mentions to the renovation team that many of the crazies return home even though the asylum has been abandoned and is in a horrific state of disrepair.
Cutting to the chase the head of the renovation team happens to be one of those patients and has brought his team out there to murder them one by one.
One of the cells in the asylum was this foreman's room back when he was held their sixteen years previously. The dorm is wall to wall with photos of his wife and baby whom he murdered.
Various parts of the film reveal the foreman is not truly an evil man but he's overran with mental illness. His good spirit is actually helplessly trapped inside his murderous body forever crying for release and forgiveness from those people he's hurt - Even his horrifically sabotaged chance at a real life with loving wife and child.
The psychiatrist's session recordings were documenting a woman whom had been in the asylum long before it had shut down. She appeared to suffer from a multiple personality disorder tripping her into speaking from different personalities while being questioned by the shrink recording her.
One personality is a young girl, another a boy and the final identity is an adult male named "Simon."
Throughout the ninth session the psych asks the woman were the young girl lives and the patient exclaims 'The tongue' as 'the young girl likes to talk a lot.'
She then reveals that the boy lives in her eyes.
The final line in the film is Simon speaking in the recording and when the shrink asks he replies (in a surprisingly deep & sickening male voice) "I live in the weak and the wounded, Doc."