I got to learn more about the Knights of Ren from Kylo Ren’s origin story comic book. Even though they did not get a prominent appearance in the movies, we did get more insight into this group of dark side practitioners in the comics.
Compared to the Sith, the Knights were weaker in the dark side, described to be clumsy, untrained, and using the dark side like a hammer. Their ideology was also more flexible and liberal than that of the Sith. The Knights of Ren believe in a lifestyle where they can do whatever they want, taking what the galaxy gave them, and consuming what the dark side sent them. Apparently, the Knights tend to find things to burn during their free time. They refer to the dark side as the “Shadow” and you need to be Force-sensitive and able to tap into the dark side to be a member. Like the Sith, joining the Knights of Ren requires sacrificing someone who is close to you. While the rest of the Knights wield primitive weapons, their leader is the only member of the group who wields a red lightsaber, which the Knights call the “Ren” and see it as a symbol of power and leadership. The Master of the Knights of Ren is the strongest of the group and traditionally holds the title “Ren” in their name.
Based on all these facts, the Knights of Ren are interesting and unique, but there is nothing special about what makes them. Unlike the Jedi or Sith, the Knights of Ren don’t have any kind of formal training in either combat or the Force. They are essentially untrained thugs with a natural affinity with the dark side. Due to this, it is very likely, and possibly inevitable, that a similar group of dark side practitioners will rise to replace them in the aftermath of Rise of Skywalker. Either that, or the Knights of Ren left behind some kind of scripture or record that will inspire others to follow in their footsteps and become the next version of the Knights of Ren. If the Knights of Ren continued to be active, I believe their mastery of the Force would grow the more they call on it, which would have eventually turned them into an even greater threat.