Star Wars: The Rise of Skywalker officially made Darth Revan, along with a group of other ancient Sith Lords, part of Star Wars canon. Revan was the main character in BioWare's Knights of the Old Republic video game, which itself is part of Star Wars Legends but has been a story that fans have wanted to see acknowledged in some way in the overarching Star Wars franchise. And now, it seems it's finally happened.
Over the years, various parts of the Knights of the Old Republic series have found their way into the Star Wars animated shows, movies, and canon video games. For instance, the Hammerhead corvettes that are seen in Rogue One: A Star Wars Story originated in Knights of the Old Republic. But more recently, it looked like a statue of Darth Nihulus, from Knights of the Old Republic II: The Sith Lords, was seen on the planet Exegol in The Rise of Skywalker. While that hasn't specifically been confirmed yet, another part of KOTOR has been now.
Star Wars: The Rise of Skywalker introduces legions of Sith troopers, who are all part of Emperor Palpatine's Final Order. While the Sith troopers get limited screentime in the movie, the official Star Wars: The Rise of Skywalker Visual Dictionary confirms they were separated into legions of 5,000 troopers. And each legion was named after an ancient Sith Lord - one of them being "The 3rd: Revan Legion". This officially confirms Darth Revan as canon in Star Wars.
While this isn't the first time that Star Wars has flirted with the idea of making Darth Revan canon, it is the first time that it has been explicitly confirmed. Furthermore, what's interesting about the classification is that the Visual Dictionary suggests "only the Sith Eternal cultists know the significance of these names." So while Darth Revan is canon, no one really knows who he is. Perhaps only Palpatine does. But beyond just naming Darth Revan, the Visual Dictionary also confirms other Sith Lords as canon as well.
The Sith Lords Andeddu, Tanis, Tenebrous, Phobos, and Desolous also had Sith trooper legions named after them. Darth Phobos and Desolous first appeared in The Force Unleashed video game, which had its story written by George Lucas himself, who wanted to use this game as a means of bridging the gap between Star Wars: Episode III - Revenge of the Sith and A New Hope. So Star Wars: The Rise of Skywalker not only brought a Sith Lord from one video game into canon but two more as well from another video game. Now it remains to be seen if Lucasfilm plans to expand their stories in any way beyond the film.