Viewers of Star Wars: The Rise of Skywalker most likely wondered what planet the evil Kylo Ren engaged in a murderous rampage on at the start of the film. The surprising answer is that it was the Sith planet Mustafar, which looked significantly different from how it first appeared in Revenge of the Sith.
In Episode III, Mustafar was depicted as a fiery hellscape where the fallen Jedi Anakin Skywalker lost to his former master, Obi-Wan Kenobi, and was severely burned and mutilated as a result. This world made a return in Episode IX as the red forest planet where Kylo Ren found a Sith Wayfinder, which helped him locate the still-living Emperor Palpatine on the planet Exegol. The reason the volcanic planet looks so different in the sequel trilogy lies with Darth Vader himself.
In Vader Immortal: A Star Wars VR Series (which takes place between Episode III and IV), the Sith Lord discovered a device called the Aeon Engine, which was created by Lady Corvax on Mustafar to harness the Bright Star crystal's power to revive her dead husband. However, this device failed, and the Bright Star released waves of energy that devastated Mustafar, terraforming it into the volcanic world seen in Revenge of the Sith.
Vader would end up repeating history by attempting to use the Engine to resurrect his dead wife, Padmé (whom he originally tried to save from death by learning the ways of the Sith). Unfortunately, a Force-sensitive descendant of Lady Corvax, whom Vader had captured to locate the Bright Star, battled him using Lord Corvax's lightsaber. Though the Sith Lord nearly succeeded in reviving Padmé, Vader's opponent ruined his plans by piercing the Bright Star with his saber, destroying it along with the Aeon Engine.
In an ironic twist of fate, the destruction of the Engine caused Mustafar to begin reverting back to the lush world it once was. This resulted in the forest of irontrees where Kylo Ren slaughtered Sith cultists at the beginning of The Rise of Skywalker over thirty years later.
Like Lady Corvax, Vader failed to use the Aeon Engine to revive his dead spouse and ended up drastically changing the planet Mustafar. For someone who oversaw an Empire notorious for destroying planets, it was surprising to see Vader inadvertently bring one back to life.