In Rogue One, we got to see Darth Vader’s personal residence. Shortly after the film was released, it was confirmed that the castle was on Mustafar, which was the volcanic planet where Darth Vader was maimed and burned alive after fighting Obi-Wan Kenobi in Revenge of the Sith. It makes perfect sense that this is where Vader would live. The reason for this is because Mustafar is where Vader’s pain and hatred was born and where he truly became Darth Vader. In addition, Star Wars Rebels revealed that Mustafar was where Jedi go to die, which made it the ideal fortress against potential Jedi attacks. I have a strong feeling that we will see more of this castle in future stories. The Knights of Ren such as Kylo Ren were devout Darth Vader fanatics so it would make sense that Vader’s castle would be their headquarters. This could also be where Supreme Leader Snoke resides. Who knows? We may get to see it more detail in either Episode VIII or IX.