In a sizzle reel for Star Wars: The Rise of Skywalker shown during D23, Rey is seen wielding a red, double-bladed lightsaber. Rey's actions in the final film of the Skywalker saga have been the source of much debate, with theories ranging from her either restarting the Jedi order, creating a brand new order of Force users, to her even abandoning the Force altogether. One thing that does seems all but certain, though, is Rey's need to either repair her previous lightsaber or construct a brand new.
In the trailer for Star Wars: The Rise of Skywalker, Rey is clearly seen with a blue lightsaber - the same one she claimed in Star Wars: The Force Awakens during her battle with Kylo Ren. That lightsaber, which had previously been used by both Anakin and Luke Skywalker, was later destroyed when it was pulled apart by both her and Kylo trying to possess it. Since she's shown with a blue lightsaber in the trailers, it's most likely she repairs the broken lightsaber or at least uses its kyber crystal to construct a new one. Either way, it's an important step in her training as a Jedi - so what would compel her to take up a red lightsaber typically used by the Sith?
The Star Wars: The Rise of Skywalker footage of Rey with the red, double-bladed lightsaber comes from a sizzle reel that debuted during the Walt Disney Studios panel at D23. Being a sizzle reel, the scene doesn't come with any context, but Rey wielding a weapon that's commonly associated with the Sith and other dark side users is worrying for sure. Not to mention, it's a double-blade and that immediately brings to mind Darth Maul, the only character ever seen in the films to use such a weapon.
During Star Wars: The Last Jedi, Rey is repeatedly tempted by the dark side of the Force, so it's possible that just like Kylo, she may find herself succumbing to the darkness within her. Then again, Rey seemingly conquered those doubts by the end of the film, proving herself to be a true Jedi. A switch to the dark side this late in the story isn't impossible, but it'd be a difficult transformation to do in a single film and give it the justice it deserves. Instead, the footage of Rey with the red, double-bladed lightsaber is most likely a tease meant to get audiences worried about what Rey might do in The Rise of Skywalker.
The scene from the sizzle reel is almost certainly an instance where Rey must take up an enemy's weapon in order to fight. The question is, then, whose weapon is it? As mentioned above, Darth Maul is who immediately comes mind for a double-bladed lightsaber, but not only was his lightsaber destroyed, he's also dead by this point in the timeline. One possibility is the weapon belongs to one of the newly unveiled Sith Troopers. Not much is known about who they are other than they're troopers loyal to Kylo Ren, but their name suggest they may have some Force sensitivity. If that's the case, then it's possible Kylo has outfitted his troopers with traditional Sith weapons - of which a red, double-bladed lightsaber would fit in perfectly. The Star Wars: The Rise of Skywalker footage is expected to be publicly released soon, at which point, we may have a better idea in what context Rey wields this lightsaber.