In Star Wars: The Rise of Skywalker, Luke Skywalker's Force ghost appears to Rey on Ahch-To to give her Leia's lightsaber, which he had kept squirreled away for years. Although the Star Wars sequel trilogy focuses on a new band of heroes, it also brings back many of the biggest faces from the original trilogy, including Luke Skywalker, Princess Leia, and Han Solo. However, the sequels take place three decades after the Rebel victory seen in Return of the Jedi, and time has changed the old guard. In Leia's case, she ultimately became a successful Galactic Senator and a great Resistance general, but not before beginning Jedi training under her brother, Luke.
After the defeat of the Empire, Leia used her newly-discovered Force-sensitivity to begin studying as a Padawan under the tutelage of her brother. While she would prove to be very gifted in the ways of the Force, Leia abandoned her Jedi training when she had a premonition that the path would eventually lead to the death of her future son. She ceased using her lightsaber at this time as well, giving it to Luke and requesting that he pass it on to one of his Jedi pupils, a promise that he would only fulfill many years later when he gave it to Rey.
Encased in a distinctive silver-and-copper hilt, Leia's lightsaber used a naturally-occurring blue kyber crystal, much like Luke's original saber that he had inherited from his father by way of Obi-Wan Kenobi. However, the leaner shape and ringed blade emitter more closely resembled the green-bladed lightsaber that Luke had constructed for himself in the interim between The Empire Strikes Back and Return of the Jedi, which makes sense, given he was the only living Jedi at the time, so his design philosophy would naturally have an outsized influence on the craft. The regal artistry and fine mother-of-pearl detailing of Leia's weapon were meant to pay homage to her adoptive roots in the Royal Family of Alderaan. Behind the scenes, this elegant feeling was achieved through the inspiration of an Art Deco lamp. The alternating copper and steel lines also bear a hint of the design seen on the hilt of Anakin Skywalker's lightsabers, both the blue saber passed on to Luke and the red one he wielded as Darth Vader.
Leia's innate skill with the Force was already hinted at in The Empire Strikes Back and Return of the Jedi, and is shown much more explicitly in Star Wars: The Last Jedi. The Force was said to make such an instrument much easier to handle (which is also potentially why Finn is hinted to be Force-sensitive in The Rise of Skywalker, having briefly but proficiently wielded Luke's lightsaber in Star Wars: The Force Awakens). Being skilled enough to best Luke in a sparring session is a testament not only to Leia's abilities as a duelist, but also to her strength as a conduit for the Force, which would continue to influence her life even after walking away from the Jedi path.
Leia's lightsaber is a wonderful bit of prop design and has loving aesthetic touches to distinguish it from the sabers that came before it, but its similarities to Luke's lightsaber are perhaps more noteworthy than its differences because, much like their twin owners, they are meant to be seen as counterparts to each other. At the climax of The Rise of Skywalker, when Rey uses both of them in tandem to defeat Darth Sidious on Exegol, it is not only the power of "all the Jedi" that aids her, but also the dioscuric weapons of two of the most important people in the history of the galaxy.