Ren's History
"Ren" was the alias of a Force-sensitive human male who led the Knights of Ren during the era of the New Republic. A user of the dark side of the Force, Ren was heavily scarred and disfigured, though made no attempt to hide the damage. During his time with the Knights of Ren, Ren sought to induct other dark side users into their ranks.
After encountering Ben Solo, Padawan to Jedi Master Luke Skywalker, during a confrontation on the planet Elphrona, Ren sensed that the Force was strong with the young Jedi, and suggested that he follow the path of the dark side. Several years later, Solo contacted Ren seeking to join the Knights of Ren after the Jedi Order fell to ruin.
At some point before the rise of the First Order and their war against the Resistance, the individual who would later be known as "Ren" came to lead the Knights of Ren, an organization of Force-sensitive warriors who followed the dark side and an ideology based on the Ren, the red-bladed lightsaber wielded by Ren. During his time as a user of the dark side, Ren became heavily scarred and burned. Traveling throughout the galaxy, the Knights of Ren searched for other Force users to join their ranks.
After hearing from their master about the Force-sensitive criminal Karrst, who, along with his brother Filin, caused over one hundred deaths, Ren and the Knights of Ren sought to locate him. Eventually finding the brothers under attack from forces of Sector Security, Ren spoke to Karrst while the Knights of Ren dealt with the security forces. After Karrst hesitated when Ren informed him that entry into the Knights of Ren required the death of his brother, Filin instead killed Karrst and requested to join in his place. However, as Filin was not sensitive to the Force, Ren was forced to kill him. Having failed to recruit Karrst, Ren chose to leave, wishing to find something to burn.
Confrontation on Elphrona
Some time later, the Knights of Ren discovered an ancient outpost of the Jedi Order on the planet Elphrona, and entered the outpost, seeking to steal the Jedi artifacts inside. However, the Jedi Master Luke Skywalker had learnt of the outpost with the help of the explorer Lor San Tekka, and also sought to retrieve the artifacts. Ren and the Knights confronted Skywalker, who had visited the outpost alongside San Tekka and his own Padawan Ben Solo. Ren declared that the artifacts belonged to the Knights, threatening Skywalker. The Jedi Master, however, wished to prevent the Knights of Ren from taking the artifacts.
Ren ignited his lightsaber, and the Knights surrounded Skywalker. However, despite being outnumbered, Skywalker managed to incapacitate the Knights, destroying their weapons. With the Knights defeated, Ren stepped forward to face Skywalker himself. Ren revealed that he had activated a kill switch on his lightsaber, which would cause it to explode if it left his hand. However, as Skywalker was too strong to handle, Ren chose to leave with the rest of the Knights. Before departing, Ren pointed out the presence of the dark side in Solo, and removed his mask, telling Solo that he could take other paths in life. Ren threw his helmet onto the ground, and told Solo to contact him if he ever wished to learn about the dark side.
Contacting Ben Solo
Ren was still in command of the Knights by 28 ABY, intending to travel to Varnak, a world in the Mid Rim. At the same time, Solo escaped a confrontation with his master and the destruction of Skywalker's Jedi temple. After meeting with Snoke, who had been speaking inside his head for many years, Solo chose to seek out the Knights of Ren. He returned to Elphrona, where Ren's mask remained within the Jedi outpost. After Solo put on the mask, Ren contacted him, and Solo asked whether the Knights would take him in, mentioning Snoke. Ren agreed, and told Solo to meet him on Varnak.
On Varnak, Ren and his Knights met with Solo in a cantina. Vicrul informed Ren of Solo's arrival and Ren greeted him. Realizing his mask made Solo feel nervous, Ren took it off. He informed Solo that he would only be accepted in the Knights if he caused "a good death." Solo responded that he had killed a Jedi, leading Ren to express his doubts if he had killed Luke Skywalker, as Snoke had told him that maybe Solo had done it. Solo told him that it wasn't Skywalker, but another Jedi, leading Ren to take a seat and ask Solo to tell how he gave that Jedi a "good death."
Solo explained that when he contacted Ren while on Elphrona, he discovered that he had been followed by Tai, Voe and Hennix, three of Luke Skywalker's Jedi pupils who had been off-world when the Jedi temple was destroyed. Ren asked Solo for details about the three Padawans. Ren did so and then told Ren and his Knights that Tai, Voe, and Hennix engaged him in a duel to interrogate him about what had really happened with the Jedi temple. While trying to escape, Solo ended up accidentally killing Hennix by redirecting his lightsaber towards him before sealing Tai and Voe inside the outpost where Ren first met him before he left Elphrona.
Having heard the whole story, Ren asked his Knights about their opinions and agreed that while Solo had caused a death, it wasn't one deemed good enough to accept him into their fold, as he hadn't really intended to kill Hennix. However, as Snoke had vouched for Solo, Ren allowed him to accompany them anyway. Before leaving, Ren remarked that Solo looked like trash with his Jedi robes and instructed Kuruk to give him a fresh set of clothes.
Death by Kylo Ren
The company headed to the Minemoon of Mimban in search of a relic called a Minesplinter, but were intercepted by a group of armed miners who had discovered the artifact earlier and sought to keep it. The Knights of Ren made quick work of the resistance but were unable to pry out any information as to where the artifact was located, so Ren decided to kill them one by one until one of them spoke, but was stopped by Ben who used for the first time his mind probing ability to find out the location of the artifact. Ren was impressed by this feat and felt Ben was starting to be useful. Now knowing where to look, Ren and his knights massacred the miners much to Ben's shock.
Making their way to the artifact, the Knights and Ben encountered more of the miners. Ren ordered the Knights to attack them, including Ben, but was disappointed in his cowardice, even with his superior Jedi training. Before making any move, the two were intercepted by Ben's former Jedi friends, who took on the Knights and Ben. With Tai going against Ben, Voe blocked Ren's killing blow against one of the miners with him gloating how he fought against Luke Skywalker years earlier, but in fact was easily defeated by the Jedi Master.
Voe fought furiously against the Knights of Ren, but couldn't defeat them all and was knocked down. With his opponents down, Ren went over to see how Ben was doing, but arrived just in time to see how Ben was unable to embrace the darkness and finish off Tai, so he snapped the Jedi's neck.
Ren was utterly disappointed in Ben, despite Snoke's high hopes for the boy, so he planned to finish off Ben, but was attacked by him instead. Ren noticed how Ben was inexperienced in real combat and hoped to finish him off quickly by tackling him down into a pit. As the two fell and fought, Ben was rapidly giving in to the Dark Side, which was sensed by Leia Organa, Rey, Snoke, and even the late Darth Sidious, with Ben giving a finishing blow to Ren, killing him and finally giving Ben his "good death." With Ren gone, Ben finished off Voe and ascended to his place as the leader of the Knights of Ren.
Legacy
While Ren's Knights were inherited by Solo, who took on the name Kylo Ren, after the skirmish on the Minemoon, he was given command of the group as a gift from his new master, Supreme Leader Snoke, after he passed a test that Snoke had given to him.
