The question isn't whether or not filmmaker M. Night Shyamalan is trying to surprise you. The question is how. Audiences were introduced to the director's proclivity for the twist ending in early successes like The Sixth Sense and Unbreakable, and his latest effort stars James McAvoy as a man with 23 different documented personalities, a few of which are holding three young women captive in anticipation of some mysterious event. There's more than one turn in the plot of the movie, but the most shocking one is a reference that falls at the very end. Spoilers ahead! The Split ending connects to Unbreakable in a very interesting way.
Split is the polar opposite of a whodunit. The audience knows from the beginning that McAvoy's character Kevin has done something terrible, even if he's keeping it from certain parts of himself. What's unclear is why some of his personalities have colluded on this scheme and what it is that they are expecting to happen. They tell their prisoners that they will serve a purpose when some "sacred" day arrives. That day turns out to mark the arrival of a 24th personality — an animalistic one whose strength and abilities are superhuman. "The Beast" spares Casey (Anya Taylor-Joy) when he realizes that she is as "pure" as he is, i.e. she's also endured abuse.
Casey is found alive in a maintenance building underneath the Philadelphia Zoo, where Kevin works. Later, the patrons of a busy diner watch a news report about the incident on TV. The perpetrator is still at large and has been given a flashy nickname that refers to his many selves: "The Hoard." A woman sitting at the counter wonders aloud if anyone else remembers another out-there criminal with a similarly strange calling card who was arrested about 15 years before. The person next to her leans forward, revealing himself to be Bruce Willis. He's wearing a uniform with the name "Dunn" and answers the woman's question: Yes, he was called Mr. Glass.
In the last moments of Split, Shyamalan links his new movie to Unbreakable from 2000. In that film, Willis plays a security guard who discovers that he is indestructible when he survives a devastating train accident. He's shepherded into superhero-ism by a man named Elijah (Samuel L. Jackson), a purveyor of comic books suffering from a disease that makes his bones dangerously fragile. Dunn discovers through the ESP he develops that Elijah has been committing atrocities for years in an attempt to find the counterpart to he supervillain he's always believed himself to be. He calls himself "Mr. Glass" and is turned over to the authorities.
It's a strange connection for Split to make and seems quite random, since no other characters but Dunn overlap and the plots don't converge anywhere else. I don't see the purpose in pointing out that Unbreakable and Split exist in the same universe unless Shyamalan is actually asserting that all of his original screenplays exist in the same universe. Though, that's immediately a problematic theory since Willis plays different characters in Unbreakable and The Sixth Sense.
Split's ending has a broader implication than any of Shyamalan's other plot twists. This one encourages a new reading of at least one other film.