Knights of Ren

"Even you, master of the Knights of Ren, have never faced such a test."

- Supreme Leader Snoke, to Kylo Ren, the Knights of Ren's leader.

The Knights of Ren are an antagonistic faction in the Star Wars sequel trilogy, first mentioned in Star Wars: The Force Awakens. It is an organization that was formed sometime after the Battle of Endor and has existed in the years since then. They were founded by Supreme Leader Snoke, but are currently led by his apprentice, Kylo Ren, and a sub-faction of the First Order. Since Kylo Ren is their leader, the organization has taken on his surname as a result.

So far, only their leader Kylo Ren has appeared, with some of the members seen in a vision by Rey.

The Force Awakens
Their leader Kylo Ren acts as the primary enforcer of the First Order, searching for a map which leads to Luke Skywalker, the last Jedi in the galaxy.

After touching Luke Skywalker's lightsaber, Rey has multiple visions, one being several Knights of Ren standing in the rain, led by their master Kylo Ren who kills an unknown/unnamed man; who had the rank of Jedi Knight.

Eventually, Kylo Ren is severely injured in a battle with Finn and Rey and called off by Supreme Leader Snoke to complete his training.

The Last Jedi
While they do not appear in the movie The Last Jedi, Luke Skywalker does tell Rey that after Kylo turned to the Dark Side, he recruited several of Luke's Jedi students, and murdered the rest. It is quite possible that the Knights of Ren were formed by these fallen Jedi and that they would go on to serve the First Order and Kylo Ren as a whole.

Trivia

 * The Ren surname as a result of the group's membership is similar to that of Darth becoming the title of all members of the Sith. Appropriate, as the Knights of Ren are in essence a new Dark Side organization picking up where the Sith left off.
 * In this respect, they could be likened to numerous Dark Side organizations in the Legends continuity, such as the Blackguards, the Reborn, the Disciples of Ragnos, the Shadow Academy, and so on.
 * Despite presumably being a Dark Side aligned organization, Kylo Ren is the only member definitively known to use a lightsaber; the rest all seem to use either blasters, staves, or other weapons.
 * According to leaked behind-the-scenes images and concept art, the Knights of Ren were originally supposed to appear in The Last Jedi and might've fought Luke and Rey on Ahch-To.
 * Director Rian Johnson also revealed that the movie was too full and there wasn't any room for the Knights of Ren to appear in The Last Jedi and that even if they did, he would've had them killed and thus spared them from such a fate.