Luke Skywalker (Mark Hamill) notably had a Sith kyber crystal necklace in Star Wars: The Last Jedi, adding more insight into his journey after the original trilogy. The search for Luke was unveiled as a priority from the opening moments of the Disney-era installments. Rey (Daisy Ridley) took it upon herself to lead the mission on behalf of the Resistance, arriving in Ahch-To to enlist the help of the legendary Jedi Master. Initially reluctant to help their efforts, Luke slowly came around by first training Rey in the ways of the Force. Though he claimed to want the Jedi to end, the red crystal fashioned into a necklace symbolized otherwise.
When his nephew, Ben Solo (Adam Driver), fell to the dark side and transformed into Kylo Ren, Luke blamed himself, believing he failed in his teachings. In turn, he became haunted for not protecting future generations. After becoming disillusioned about the Jedi Order, Luke forced himself to live in exile on Ahch-To. Meanwhile, the rest of the galaxy was swept up in another deadly war between the Resistance and the First Order. With Luke's immense knowledge and connection to Kylo Ren, he was one of the keys to defeating the First Order. He may have become one with the Force by the end of The Last Jedi, but his physical and spiritual legacy propelled the Resistance in the right direction.
The search for Sacred Jedi texts was a major element of Luke's journey after defeating the Empire. The significant objects were kept on Ahch-To and shown to Rey by Luke during her initial training. During the Ahch-To sequence, Luke unveiled a fragmented red kyber crystal presented as a necklace. Before the movie's release, theories suggested the crystal belonged to Luke's father, Anakin Skywalker (Hayden Christensen), who fell to the dark side as Darth Vader before redeeming himself. That wasn't the case as the Sith objected was later revealed to be a Jedi Crusader pendant, which Luke kept because it carried powerful symbolism in the conflict between the Jedi and Sith in Star Wars lore.
As confirmed in Star Wars: The Last Jedi's Visual Dictionary, Luke's red kyber crystal was a relic once owned by a Jedi Crusader. The pendant featured a broken piece of an ancient Sith lightsaber crystal, which Crusaders wore as trophies after defeating their Sith enemies. The act was done when the Sith were numerous, suggesting the objects were centuries old. The Last Jedi and Star Wars: The Rise of Skywalker explained how much time Luke spent traveling the galaxy to acquire knowledge centered around the Jedi. In doing so, he encountered ancient artifacts. The red kyber crystal was one of those symbolic relics. While it was presumed the necklace disappeared when Luke become one with the Force, Rey took possession of the ancient texts.
Rather than keep the Jedi Crusader pendant hidden on Ahch-To, Luke elected to display the pendant. This was a significant move since it suggested Luke viewed the Sith object as his own trophy. Not only did it symbolize Luke's past in defeating the dark side, but it proved the Jedi Master still had an interest in building a future for the light side. This notion was reiterated by Luke when he directly assisted the Resistance in Star Wars: The Last Jedi. His teachings, findings, and artifacts also gave Rey the knowledge and motivation she needed to the darkness led by her grandfather, Emperor Palpatine, before heading into the unknown as the last Jedi.
