Star Wars loves to reuse locations in the galaxy. From The Mandalorian taking a detour to Tatooine to D'Qar showing up in Star Wars: The Last Jedi, one of the easiest and best callbacks to tie the giant franchise together is location. (Real estate agents in the galaxy must be thrilled.) Star Wars: The Rise of Skywalker features tons of planets, but there's one, in particular, that is one of the best Star Wars: The Rise Of Skywalker easter eggs in the entire film.
Warning: Spoilers for Star Wars: The Rise Of Skywalker follow.
The first 45 minutes or so of the final Star Wars installment is crammed to the gills with planets. Between Kylo Ren, Poe, Finn, and Rey, there are no less than six planets visited in under an hour. (This is one of the reasons that opening hour feels so intensely busy: Travel!) Five of these places are brand new to the series. Kylo goes to Exegol, where the Emperor is living. Finn and Poe start in the Sinta Glacier Colony before heading back to Ajan Kloss, where Rey is training. From there, Poe, Finn, and Rey go first to Pasaana where they find the dagger, and then to Kijimi.
But there's one more location, which no one gives the name of, but it's somewhere fans have been before. That's the planet Kylo Ren starts on, where he tracks down the wayfinder to take him to Exegol in the first place.
Fans have been to Mustafar before, in two different Star Wars movies and a Star Wars TV series. As is evidenced by Kylo Ren's fight, it's a planet without a lot of sunlight, kept warm and running by the massive amount of volcanic activity and lava jets and rivers that flow across the surface.
It's a famous planet in Jedi lore, because it was here, in 19 BBY, where Anakin Skywalker was defeated by Obi-Wan Kenobi in single combat and left for dead. It was also where the Emperor found Anakin after the fight and remade him into Darth Vader. Mustafar then became the location of Vader's castle, where he would retreat during his downtime. (Makes sense, this is where he was rebuilt.) Both the planet and the castle were featured in Rogue One: A Star Wars Story.
But fans also visited this location with Skywalker and Kenobi during the Star Wars: The Clone Wars TV series. Set prior to Vader's transformation, the duo traveled there to a Sith facility, where Force-Sensitive children had been kidnapped, with plans to turn them to the Dark Side.
Mustafar is clearly not a popular place to build a summer home, especially with Vader as a crabby next-door neighbor. But as his original home base, it makes sense Vader would have had one of the few wayfinders to Exegol. Naturally, it's in the ruins of the castle that Kylo Ren finds it.