According to a post on We Got This Covered, The Knights of Ren will be the main villains in Star Wars episode 9.
The Knights of Ren are a group that intrigued the fandom when we saw them in Rey's vision in episode 7, and people felt cheated when Rian Johnson didn't use them in The Last Jedi.
People have short memories because the fake story some less reputable YouTube channels were selling was that the Knights of Ren were going to likely be killed by Luke Skywalker and Rey or executed by Kylo Ren for failing to capture Luke and Rey.
Rian Johnson said that if he had used the Knights of Ren, they would have been in Snoke's throne room, and we know what happened to the Praetorian Guard.
Anyway--after the Knights of Ren were featured in the poster that surfaced two weeks ago-- there is a lot of buzz around this group and what their role will be in the final installment of the Skywalker Saga.
So I'm going to chime in on this topic and tell you what I think the Knights of Ren will be in episode 9--along with another group inspired by something from Legends that you might not even know about.