WARNING! This Post Contains MAJOR SPOILERS!!!
As reactions to Split continue to spread, fans are discussing the reveal that this film is actually set within the same universe as Shyamalan's 2000 hit film, Unbreakable. With Split ending with an official cameo from Bruce Willis, it's clear that there's some important connective threads being woven between the two.
With that in mind, We explain Split's Connection to Unbreakable!
Unbreakable was the story of a "real-life" superhero coming to accept his extraordinary nature, and facing the responsibilities that come with it. By comparison, Split is the story of a supervillain's rise to power.
Whereas David Dunn was reluctant about revealing his powers and using them, Kevin/Dennis/Patricia/Hedwig/Beast (aka "Horde") is actively seeking access to his. Whereas David ultimately uses his powers to be a natural protector of humanity, Horde has now been emboldened to use his power for destructive ends, literally consuming the "unbroken" members of society.
With David Dunn now aware of Horde's existence, he will likely feel compelled (as possibly the only one powerful enough to stop him) to do something about it.
Shyamalan revealed in an interview with EW, that Kevin/Horde is a character he wanted to put in the first Unbreakable.
According to the filmmaker:
"This character, Kevin from Split, was in the original script of Unbreakable. The original draft of Unbreakable focused on David Dunn and Elijah as his mentor. Elijah tells him, “You’re a comic book character, go try it.” And instead of bumping into the Orange Suit Man, David bumps into one of Kevin’s personalities and goes to save the girls. So you’d have been watching the girls side of it the whole time. That was the outline...
Some of the Kevin Wendell Crumb [James McAvoy’s character in Split] scenes were already completely written, all the way back then. One of the “Patricia” scenes, the Hedwig introduction scene, those were written over 15 years ago. I have them written by hand in my notebooks."
Watching Split a second time - with the knowledge that it is a tie-in story to Unbreakable - definitely reframes the storylines, character arcs, and themes of the film.
Both Unbreakable and Split examine some common themes, as means to explain how superheroes (or villains) could exist in real life; the common theme between them, is that trauma is what catalyzes humans to evolve and reach new levels of potential.
In Unbreakable, David Dunn realizes the extraordinary nature of his power in two separate traumatic accidents (the train crash, and car crash when he was younger); similarly, David's nemesis, Elijah "Mr. Glass" Price (Samuel L. Jackson), had his villain origin tied to the traumatic childhood illness that made his bones fragile. Split antagonist, Kevin, developed the Dissociative identity disorder that gave birth to The Beast after suffering abuse as a child - suffering that helps him bond with his would-be victim, Casey Cooke, when it's revealed she's a childhood abuse victim, as well.
This theme isn't all that original in superhero stories (Deadpool used it), but Shyamalan definitely employs it in a much different way for his Unbreakable universe, using superhuman emergence as metaphor for rising above personal trauma. However, as a plot device, this phenomenon established in both Split and Unbreakable leaves the door open for many other potentially great superpower origin stories (or match ups) in the Unbreakable universe.