"Split" is generally regarded as M. Night Shyamalan's return to form after several exceptionally bad movies ("Lady in the Water," "The Happening," "The Last Airbender," and "After Earth"), although it was preceded by 2015's "The Visit" which has received "mixed to positive" reviews.
The movie opens quickly with three young women (Anya Taylor-Joy, Haley Lu Richardson, and Jessica Sula) being kidnapped by James McAvoy's character. As is made clear by the trailer and the poster, he has a fairly large set of personalities, so that part's not a surprise unless you're coming to the movie with no fore-knowledge whatsoever. So his behaviour after locking them up isn't much of a surprise - although McAvoy puts in a good performance, so it's definitely creepy.
We see some of McAvoy's visits to his psychologist (psychiatrist? who knows - played by Betty Buckley) - which allows the movie to lay out her beliefs about the possible abilities of people with dissociative identity disorder who seem to be able to change their own body through belief. And we also see pieces of Casey's (Taylor-Joy) increasingly unpleasant childhood.
Between these two things, Shyamalan laid out the plot a bit too clearly and I had a very good idea where he was going with most everything by the end. He's been known in the past for pulling extraordinary twists at the end, having embedded hints throughout the movie. Hardly anyone catches them. If there was a twist that I wasn't supposed to see ... well, I got all the big ones. And I didn't find it emotionally involving or scary, just a bit creepy and unpleasant. A good performance from McAvoy wasn't enough to pull this one through.