There is a lot of mystery surrounding The Rise of Skywalker. J.J. Abrams and Lucasfilm are doing an excellent job of keeping things under wraps. With that being said, they still won't even reveal what species the Knights of Ren are. We know next to nothing about these epic looking villains, other than the fact they fight with Kylo Ren. Rian Johnson chose to leave them out of The Last Jedi, but Abrams has brought them back and they are featured prominently in The Rise of Skywalker promotional material.
A new reference guide titled Ultimate Star Wars New Edition has shed some light on the Knights of Ren. Actually, it's a very small amount of light, more like a sliver of light. But, it proves that The Rise of Skywalker might not give us much more information on them, or if it does, it's going to be a big part of the plot. You can read about the Knights of Ren below.
"The Knights of Ren are Kylo Ren's most deadly and mysterious servants. With their bodies encased in rusty, battered armor and their faces permanently concealed beneath ominous masks - echoing that of the new Supreme Leader himself - even their species remains a mystery. What is obvious to anyone who witnesses them in action is that their fighting skills and martial prowess are without equal. Each Knight is armed with a lethally effective and unique weapon, suited to either long range or close-quarter combat."
The Knights of Ren are not Sith and they are not tied to The Empire and Darth Vader. Additionally, they do not serve The First Order either, as they are tied exclusively to Kylo Ren, however, they may have had ties to Supreme Leader Snoke. They are "a new generation of dark side users that emerged to fill the void left by the demise of the last Sith Lords, Darth Sidious and Darth Vader." It is unclear if they are schooled in the ways of the Force at this time, though we should learn soon enough.
J.J. Abrams has said that the Knights of Ren do have a backstory, but it has not been revealed. It's possible The Rise of Skywalker will go into this story, but there is already a lot of ground to cover, which is why Rian Johnson did not include them in The Last Jedi. "I guess I could've used them in place of the Praetorian guards but then it would feel like wasting them because all those guards had to die," said Johnson when asked why they weren't included in the movie.
Since J.J. Abrams and Lucasfilm are doing such a good job keeping The Rise of Skywalker details a secret, Star Wars fans have been spreading speculation and rumors like wildfire. There's a lot to live up to, but even if we don't get the Knights of Ren backstory in the upcoming movie, there's still a chance we'll learn about them down the line in a possible novel, comic, or maybe even a Disney+ series. You can head over to DK to get more information on ordering the Ultimate Star Wars New Edition book.