The ending of Split revealed that it and Unbreakable take place in the same cinematic universe, a secret that he went to great lengths to protect. Shyamalan later said that it was "always my dream to have both films collide in this third film."
Bruce Willis from Unbreakable is coming back as David Dunn, alongside Samuel L. Jackson's Elijah Price AKA Mr. Glass. Split star James McAvoy will also star in Glass, again playing Kevin Wendell Crumb and his many different identities. Additionally, actress Anya Taylor-Joy's Split character, Casey Cooke, will be in Glass.
In terms of the story, Collider reports that Glass follows Dunn as he's going after Crumb's most extreme personality, The Beast. The movie will chronicle a "series of escalating encounters, while the shadowy presence of Price emerges as an orchestrator who holds secrets critical to both men."
Glass is slated to release on January 18, 2019. Shyamalan first teased the film back in February when he was working on an outline for it. Originally, McAvoy's character--pre-multiple personalities--was going to appear in Unbreakable but was ultimately cut. Split was Shayamalan's biggest opening to date, grossing $100 million its first weekend.