One of the most intriguing aspects of Star Wars: The Rise of Skywalker is that, at one point, Rey wields a double-bladed red lightsaber, and here's a complete explanation for that. As the final installment in the long-running Skywalker saga that began with George Lucas' original Star Wars film over 40 years ago, Star Wars: The Rise of Skywalker - written by Chris Terrio and J.J. Abrams, and directed by Abrams - wraps up the epic story that spans three Star Wars trilogies.
To bring the Skywalker saga to a fitting end, though, one that pays homage to - as well as pays off - all eras of the Star Wars franchise, Lucasfilm brought back Emperor Palpatine, introduced Sith troopers, and highlighted quite a tantalizing duel between Rey and Kylo Ren on top of the wreckage of the second Death Star. But to fan the flames, Rey is shown holding a double-bladed red lightsaber while wearing a dark, Sith-type of a cloak, thereby hinting at her possible turn to the dark side of the Force.
Similar to Luke Skywalker in the original Star Wars trilogy (until Return of the Jedi), Rey never made her own lightsaber. Instead, she chose to use Anakin's blue lightsaber, which was recovered in Star Wars: The Force Awakens. But in Star Wars: The Rise of Skywalker, she does appear with a new weapon: a double-bladed red lightsaber. It appears in two forms; one in which the two blades are parallel and attached to the same hilt, with the other being a simple, double-bladed form. In fact, it's very similar (if not exactly the same) as the lightsaber used by the Temple Guards, as shown in the Star Wars Rebels clip below.
Ever since Star Wars: The Force Awakens released, many fans have felt that Rey would be better suited for a double-bladed lightsaber since she was already so well-trained to use her staff, but then she trained once again with Luke in Star Wars: The Last Jedi so that she could properly wield Anakin's lightsaber as well as harness the power of the Force. And so, Rey's double-bladed red lightsaber appearing in Star Wars: The Rise of Skywalker is both a resolution and twist on her character arc.
Given that Rey fully believes in the light side of the Force, it's theoretically possible that Rey's double-bladed red lightsaber, which heavily resembles her staff in design, appears in a Force vision, similar to the one she first had in Star Wars: The Force Awakens, when she saw Luke's Jedi temple and the Knights of Ren. With Palpatine back in the picture, it shouldn't be surprising to see Rey experiencing a "what if" scenario through the Force in Star Wars: The Rise of Skywalker, unless she truly does turn to the dark side.