Split is an amazing movie written and directed by M. Night Shyamalan and based upon the real story of Billy Milligan, a man who suffered from the dissociative identity disorder and sentenced to prison after the rape and murder of 3 students.
The main actor is James McAvoy and his performance is very excellent in this movie. It is surely hard to play so many and different characters but he succeeded in it, well done!
Of course this is a movie, so the mental disorder is amplified with nearly superhuman abilities for Kevin, the main character. But it is still interesting, a very realistic movie would have been boring.