In an interview with Esquire , Abrams elaborated on the Sith-like group (of whom Kylo Ren was once the leader), and why he simply had to bring them back for The Rise of Skywalker.
"Theyre characters that, when we came up with them in Force Awakens and had very brief sighting of them, it was something that we had a lot of ideas of sort of further adventures with them, backstory, you know, and all sorts of things that were not the focus of the central story, and never made it into the movie," he said. "And Episode VIII, The Last Jedi, didnt address that, at all, and it just allowed for us to bring them back in."
One fan theory suggests that the Knights of Ren might actually hold the key to unlocking Rey's true identity, although it remains to be seen whether that will have any bearing on the way they are portrayed on-screen. And just because they're coming back, doesn't necessarily mean we're going to find out everything about them: Abrams also confirmed that the Knights will keep their masks on throughout the movie.
"I mean, theyre best kept more mysterious than familiar," he said, "which is just to say there arent going to be a lot of scenes with them taking their masks off and hanging out and eating sandwiches, but it felt like I definitely wanted to see more of them than we had, and I felt happy that we got a chance to do that in this movie."