In case you haven't played Vader Immortal but have it on your list (as you should), be advised that spoilers follow. The game's narrative reveals that Mustafar was once home to a Force-sensitive named Lady Corvax and her husband, who lived in harmony with the native population. The husband, a warrior known as the Black Bishop, was killed in battle — a development which Corvax was unable to accept.
She stole the Mustafarians' most sacred artifact: the Bright Star (which may or may not have been a Kyber crystal) with the intention of using it to bring the Black Bishop back to life. It worked, to an extent — Bishop returned, but was caught between worlds. The most notable effect her theft had, though, was on the planet itself. The energy released when Corvax used it for her ill-advised purpose devastated the planet's surface, transforming it into the wasteland we saw in Revenge of the Sith.
Some time after Vader's loss to Kenobi — the year 0 BBY, in fact, the year in which Star Wars: A New Hope takes place — Vader had a castle built on the planet, making it his home. Some time during the events of the original trilogy, Vader Immortal revealed that he had attempted (with assistance from a descendant of Corvax) to use the Bright Star to bring his dead wife Padme Amidala back to life, but the attempt failed, and the Bright Star was destroyed. Its destruction broke the dark curse gripping the planet, and slowly, its ecosystem began to reset itself.
It is in the midst of this recovery that we caught up with Mustafar in Rise of Skywalker, and those dudes that Kylo laid a whipping on in that opening sequence were actually Sith cultists sworn to protect the ruins of Vader's castle and its artifacts. It just so happened that Kylo had a use for one of those artifacts — the Wayfinder — and he wasn't in the mood to let a bunch of Vader worshipers get in his way.
