The Knights of Ren are an antagonist faction in the Star Wars sequel trilogy, first mentioned in Star Wars: The Force Awakens. It's an organization that was formed sometime after the Battle of Endor and has existed in the years since then. They are currently led by 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 hallucination 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. Kylo is severely injured in a battle with Finn and Rey and called off by Supreme Leader Snoke to complete his training.
After touching Luke Skywalker's lightsaber, Rey has multiple visions, one being several Knights of Ren standing in the rain, led by their second-in-command Kylo who kills an unknown/unnamed man; who had the rank of Jedi Knight.
Other than in the hallucination, the Knights of Ren have yet to make an appearance in the movies.
- 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 in the prequel trilogy.