The lava planet known as Mustafar makes an appearance early on in Star Wars: The Rise of Skywalker but looks quite different than it had during its first appearance in Star Wars: Episode III - Revenge of the Sith – stepping into the less mainstream parts of the franchise's mythology, however, the answers actually reveal themselves. Recently, the volcanic world has become an increasingly important part of the overarching Star Wars narrative, as Rogue One: A Star Wars Story revealed that Darth Vader, living in a meditative dark castle, had turned it into his his home.
In The Rise of Skywalker, Kylo Ren (Adam Driver) finds himself on Mustafar – which may be unrecognizable, given its succulent forests of trees and foggy red sky – while attempting to infiltrate the hidden planet of Exegol. As it turns out, Vader had possessed one of the two Sith Wayfinders capable of locating the planet, and after scouring the ruins of the former Sith lord's castle, Kylo Ren finds what he is looking for. In a blazing opening sequence, Kylo Ren can be seen butchering several attackers.
Oculus VR has released the third chapter of their Star Wars virtual reality game experience, Vader Immortal, and it reveals why the terrain and the environment on Mustafar has changed so drastically. In detailing the planet's extensive history, the game reveals that the planet was actually once a flourishing garden world, ruled by a force-sensitive being called Lady Corvax. When Corvax's husband was killed in battle, the lady became obsessed with discovering the powers of immortality and resurrection. In her grieving state, she stole a relic known as the Bright Star and installed it into an engine she believe would bring her husband back to life. However, the misuse of the planet's relic, in turn, converted Mustafar into the volcanic, hellscape Star Wars audiences are more than likely familiar with. But by the time The Rise of Skywalker happens, Mustafar had begun to heal and transform back into its original state.
Taking place shortly before the events of Rogue One: A Star Wars Story, Vader Immortal sees the titular character consult one of the last living descendents of Lady Corvax, a Mustafarian smuggler. Though Darth Vader is able to obtain the Bright Star from her, which he hopes to use to revive his deceased wife Padme (played by Natalie Portman in the prequel trilogy), the smuggler ends up destroying the Bright Star, sending a burst of energy throughout the planet – energy that would help begin to restore the planet.
As can be seen in Star Wars: The Rise of Skywalker, this healing process appears to be well underway. The soil, now fertile, has begun to host a plethora of iron trees, a process that is more than likely to continue. That being said, given that J.J. Abrams' latest film is the concluding piece of the Skywalker saga, it's unclear if and when audiences will return to Mustafar. Perhaps, one day, it will return to its former, flourishing, fertile, and forested glory.