While the Star Wars sequel trilogy has made a compelling antagonist out of Ben Solo / Kylo Ren, fans have admittedly been deprived of learning more about Ben's dark brothers in arms, The Knights of Ren. After getting an epic tease of the group in The Force Awakens, many fans thought Star Wars: The Last Jedi would reveal them in full - but alas, this was not the case. Just one of several fan theories that were subverted by director Rian Johnson.
Right now, The Knights of Ren remain shrouded in mystery, with answers only likely to come with Star Wars: Episode IX, or some kind of tie-in project (novel, comic book). Until then, however, there continue to be a lot of fan theories about the Knights of Ren - including this one below, which tries to build the theory that the Knights of Ren are actually the Star Wars version of The Knights of the Round Table.
The Knights of Ren are to Kylo what the Knights of the Roundtable are to King Arthur. And their disappearance means they are on some kind of quest. from starwarsspeculation
As the theory breaks it down, The Knights of Ren are very much a dark side reflection of the Knights of Camelot. Like the Camelot heroes, The Knights of Ren were formed around their belief in the bloodline of a mythic hero (Ben Solo), and that hero's ability to bring peace to the realm. The Snoke/Merlin comparison is also very fitting, as Merlin's magic and wisdom protected Arthur, the same way Snoke's Force abilities and knowledge forged the Kylo Ren.
However, the most interesting pat of this theory is what it spells out for Star Wars: Episode IX. As is pointed out, Arthur's knights were sent on many a quest in their time, even without their king; this could be the very same case we're seeing in the sequel trilogy films, with The Knights of Ren off completing some quest (collecting Force relics or conquering some kind of important locale), or completing their Force training (learning the dark side and/or forging their lightsabers). That would set up an epic debut for The Knights of Ren in Episode IX, as the personal guard for Supreme Leader Kylo Ren, with future spinoffs dealing with the origin story of the Knights, or filling in where they were during the events of Episodes VII and VIII.
Hopefully, we get something more than what we've already seen, and The Knights of Ren don't become another empty red herring of the Sequel Trilogy.