The red-bladed lightsaber known as the Ren was used by a Force-sensitive human male also called Ren. He acquired both the lightsaber and his name from his predecessor. This Ren lightsaber symbolized leadership within the Knights of Ren, a group of dark side marauders who made use of the dark side of the Force. After Ren's death at the hands of the Jedi Ben Solo on the Minemoon, the lightsaber was discarded. Ben Solo, who later took control of the Knights as Kylo Ren, wielded a red-bladed crossguard lightsaber.
The name "Ren" was given to both a Force-sensitive human male known as Ren and his red-bladed lightsaber. Ren's ideology, based on the lightsaber, viewed the Ren as simply "its nature, and nothing else," lacking remorse, concern for right or wrong, or the goals it might achieve. The lightsaber was equipped with a self-destruct mechanism activated by a kill switch, detonating the weapon if it left his hand. Ren claimed the resulting explosion could destroy everything in its immediate vicinity. The hilt featured four bent prongs at the bottom and multiple small blocks encircling the emitter ring.
The lightsaber called "the Ren" was the weapon of Ren of the Knights of Ren. A human male eventually seized Ren's lightsaber and name, rising to become the Knights' leader.
During the war between Crimson Dawn and the Sith, Ren engaged in a lightsaber duel against Sith Lord Darth Vader at Fortress Vader on Mustafar. The Dark Lord of the Sith noted the red color of Ren's blade and declared him unworthy of it during their duel. Ren defended his right to wield the lightsaber, stating he had earned it by taking it from its previous owner. Although injured by Vader's lightsaber, Ren and the surviving Knights escaped the fortress.
At one point, Ren used the lightsaber to illustrate the Knights' philosophy to Karrst, a Force-sensitive criminal being considered for recruitment. However, Filin, Karrst's brother, killed him in an attempt to take his place among the Knights. While Ren approved of Filin's actions, he lacked the Force connection of his brother and was killed by Ren.
During the New Republic Era, Ren used the lightsaber while fighting Jedi Master Luke Skywalker, his apprentice, Ben Solo, and Lor San Tekka in 19 ABY. By 28 ABY, Solo embraced the dark side of the Force, killed Ren in a lightsaber duel, and then used the fallen dark knight's lightsaber to kill the Jedi apprentice Voe. Solo abandoned Ren's lightsaber next to Voe's body on the Minemoon, and transformed his blue-bladed Jedi lightsaber into a red-bladed crossguard Sith lightsaber that he used as Kylo Ren—the master of the Knights of Ren and later Supreme Leader of the First Order.