Unfortunately, in Star Wars: The Last Jedi we were not given anything on the Knights of Ren, but they could be back in Episode IX.
In Star Wars: The Force Awakens we are briefly introduced to the Knights of Ren, a band of dark side sympathizers that are under the leadership of Kylo Ren. These characters were not present in Star Wars: The Last Jedi, but there could have been mention of them.
The following piece of information will have certain spoilers about Star Wars: The Last Jedi. So, if you haven’t seen the movie yet (what are you waiting for?), please disregard.
We are given some startling and vivid recollection of Ben Solo’s descent to the dark side in the movie. At first, we have Luke’s initial retelling of the scene which is just that he confronted him. We later find out that this wasn’t correct from Kylo Ren.
Kylo tells a different version that has Luke trying to murder him. After confrontation from Rey, Luke tells the real story, which is similar to Kylo’s. Luke mentions that he simply couldn’t bring himself to strike Ben down.
Luke tells us that Ben and some of the other Jedi trainees turned and killed the rest and destroyed the temple. With these other trainees turning to the dark side as well there really is only one logical conclusion. That is that they became the Knights of Ren. Aside from that, the only other scenario I can think of would be Snoke killing them.
With Snoke out of the picture, it would make sense that if they are the Knights of Ren and that they will be featured in Episode IX. I really am hoping for this to be the case. As much as I enjoyed Star Wars: The Last Jedi, I thought we were lacking in force user battles. With what appears to be a time jump coming in Episode IX, it seems we may have more force users on both sides.
This would be a great way to bring some life back to the franchise as a whole as a lot of people were disappointed with The Last Jedi and wanted to see more light saber action. For more on the Knights of Ren, I would highly suggest you check out Star Wars HQ’s video on the possible reemergence of the Knights in Episode IX.