Ren's knights were mentioned repeatedly in the Star Wars sequel trilogy, but they didn't play a really big role. A comic series explains what this group is all about and how Kylo Ren became its leader.
Who exactly were the Knights of Ren and how did Kylo Ren become their leader? They have that Starwars-Films episode 7 through episode 9 are not really cleared up. If you want to find out the secret, you have to read the comic series The Rise of Kylo Ren.
Caution! Spoilers follow here!
The comic series The Rise of Kylo Ren comprises four editions. These tell how Ben Solo met the Knights of Ren for the first time and how he eventually became their leader.
It becomes interesting after Ben believes that his master Luke Skywalker betrayed him that the Jedi Academy was destroyed and escaped. He already knew the Supreme Leader Snoke at that time and went to him. Snoke sends him and the Knights of Ren to the moon Mimban to get an artifact. Meanwhile, the surviving apprentices from Luke, Voe and Tai go looking for Ben. They meet him on Mimban and call him a murderer because they think he killed Luke.
Ren, the then leader of the Knights of Ren, kills Tai because Ben is unable to do so. But when he mocks Ben, there is a fight between the two. Ben Solo defeats Ren and kills him when, on Snoke's telepathic instruction, he surrenders to the Dark Side of the Force. Then he murdered Voe. These two acts earn him the respect of the other Knights of Ren, who appoint Ben as their new leader. At the end of the comic series, Kylo Ren, as he now calls himself, constructs his iconic lightsaber. To do this, he converts his blue Kyber crystal into a red one, as Sith apparently always does.
As the comic series Star Wars: The Rise of Kylo Ren shows, other media are required in addition to the films to understand the complete history of the sequel trilogy.
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