An age-old Sith cave, acting as a locus for the dark side of the Force, was situated far beneath the surface of Mustafar, a planet known for its volcanic activity in the Outer Rim Territories. Constructed by the ancient Sith as a Sith shrine in their pursuit of the secrets of eternal life, this cave was positioned along the path of one of the planet's lava rivers, specifically at the location of a previous Sith temple within the Gahenn Plains, and it resonated strongly with dark side energies.
Not long after the rise of the Galactic Empire, Darth Vader, the newly christened Sith Lord, journeyed to this Sith cave. His purpose was to harness its energy to bleed a kyber crystal for the creation of his own lightsaber. Later, Vader made the decision to erect a fortress directly above the cave. This fortress was intended to channel the power of the dark side, with the ultimate goal of resurrecting his deceased wife, Padmé Amidala. Utilizing designs he had acquired from the ancient Sith Lord Darth Momin, whose spirit was confined within an artifact, Vader built a colossal obsidian castle over the cave to serve as his private residence.
The construction of the castle triggered a series of natural disasters across the planet, which incited the native Mustafarians to initiate an attack against Vader. While the attack was unsuccessful, it provided Momin's spirit with an opportunity to create a portal through the Force inside the Sith cave and summon his physical form while Vader was preoccupied with the Mustafarians. However, Momin's revival was short-lived, as Vader eliminated the ancient Sith Lord and then proceeded through the portal himself. Although he was unable to bring Amidala back to life, the visions Vader encountered during his journey through the portal enabled him to come to terms with his destiny.
Serving as a locus of the dark side of the Force, an ancient Sith cave was located on the volcanic Outer Rim Territories planet of Mustafar, specifically within the Gahenn Plains. The cave, which housed a Sith shrine, was situated far below Mustafar's fiery surface, and a lava river flowed through it, cascading out as a lava fall. The interior of the cave was adorned with various markings on its walls, and a circular rock table was positioned at its center.
The Sith cave, situated near the Klegger Corp Mining Facility, occupied the site of both a former Sith temple and an ancient castle that belonged to Lady Corvax, a powerful Force-user who was obsessed with immortality long before the reign of the Galactic Republic. Corvax's pursuit ultimately resulted in the devastation of Mustafar, transforming the planet into a volcanic wasteland. The legends surrounding Corvax's story subsequently drew the ancient Sith to the ruined world, where they constructed the Sith shrine in an attempt to uncover the same secrets of eternal life.
In 19 BBY, shortly after the formation of the Galactic Empire, Darth Sidious, the Sith Lord, instructed his newly christened apprentice, Darth Vader, to steal a Jedi's lightsaber to create one for himself. After obtaining the lightsaber of Jedi Master Kirak Infil'a, Vader, following his Master's instructions, traveled to Mustafar, where he had previously suffered severe burns during his lightsaber duel with his former Jedi Master, Obi-Wan Kenobi.
To complete the creation of his new lightsaber, Vader needed to utilize the dark energy of the Sith cave to bleed the stolen lightsaber's green kyber crystal. This involved corrupting the crystal with the power of the dark side, causing it to turn red. After placing the crystal on the cave's rock table, the markings on the walls began to glow, and Vader experienced a Force vision in which he rejected the dark side, killed Darth Sidious, and rejoined Kenobi. By embracing the darkness once more, Vader broke free from the vision and successfully bled the kyber crystal.
Later, impressed by the dark power of the Sith cave and its potential, Vader decided to construct his own fortress above the dark side locus, establishing Mustafar as his stronghold. Sidious approved the Sith Lord's request, suggesting that Vader might be able to resurrect his deceased wife, Padmé Amidala, by harnessing the dark side energies there. The Sith Master, believing it could aid Vader's quest, also instructed him to take the ancient mask that had belonged to Darth Momin, a Sith sculptor who defied the Sith's destructive legacy, preferring to create artworks that evoked pain and fear. This Sith artifact also contained Momin's life force, trapped within it for centuries.
In 14 BBY, Vader arrived on Mustafar aboard his late wife's royal starship, accompanied by Alva Brenne, the Chief Imperial Architect, and her assistant, Roggo. While Vader was meditating alone inside the cave, Roggo was possessed by Momin's mask and murdered Brenne. Upon Vader's return to the starship near the cave's entrance, he discovered Roggo, wearing the ancient mask, working on the fortress's design. The Sith Lord then used his lightsaber to kill the possessed assistant and retrieved the hologram of the preliminary design.
Subsequently, Vader took the mask into the cave and used the Force to activate it. As the mask's eyes glowed red, Momin's trapped spirit recounted his story to Vader—his sculpting philosophy that he developed as a youngling, his training in the dark side by Sith Lady Darth Shaa, and his ultimate defeat by the Jedi before he could complete his "masterpiece." In doing so, the spirit tricked Vader into unconsciously wearing Momin's mask instead of Vader's own helmet. Upon regaining his senses and removing the mask, Vader sought out an alternate host for Momin by attacking a nearby Mustafarian patrol, capturing a Northern Mustafarian named Sssp, and forcing the captive to wear the mask.
Vader and the possessed Mustafarian soon returned to the Sith cave, where Momin proposed that his design for the fortress would finally allow him to create the "masterpiece" he had always desired, and that it would also unlock the locus's dark power, capable of resurrecting Vader's wife and bridging the gap between life and death. Vader used the Force to choke the possessed Mustafarian as a warning against betrayal, but nonetheless permitted Momin to proceed with his design.
An Imperial construction site was then established around the Sith cave. Magma troopers, AT-ACT and AT-ST walkers, and TIE fighters protected the area from lava fleas, which had been attacking the site since the initial construction attempt. One magma trooper speculated that this was because the location served as the creatures' breeding ground. After eight unsuccessful designs, each causing powerful lightning storms and lava eruptions that destroyed the construction, Fortress Vader was finally completed in 12 BBY.
In the final design, several rectangular, inclined stones were arranged around the table at the cave's center, serving as the focusing chamber for the local dark side energies. The massive obsidian tower, designed to channel the power of the dark side, was divided in the middle to resemble tuning forks and harnessed Mustafar's lava for energy.
Positioned in front of the table inside the cave, Vader used the Force to activate the castle, and a burst of light in various shapes shot from the top of the tower and up through the sky. Although he successfully opened a portal through the Force, Vader's effort was interrupted by Captain Junus, the commander of the castle's Imperial garrison, who reported via comm that they were under assault by a large army of Mustafarians. When Vader left the cave to investigate, Momin activated the castle and used its power to provoke a lightning strike on Vader outside. Momin then opened the portal for himself and summoned his physical body, reclaiming his mask from its possessed host.
Upon seeing Vader join the battle, Father Kkkt and two other Force-sensitive Mustafarians combined their strength to flood the battlefield with lava, killing both Imperial and Mustafarian combatants. As the sole Imperial survivor, Vader retreated to the castle while the remaining natives launched their final assault to destroy the dark fortress. Back inside the Sith cave, the Sith Lord activated the castle to erupt the lava surrounding the fortress, decimating the rest of the Mustafarian army.
Momin then revealed his resurrection to Vader, and the two Sith engaged in a lightsaber duel inside the cave. Although Momin claimed that the dark side would prevent Vader from reaching the other side of the portal and seemed to have the upper hand after cutting off Vader's lightsaber hand, Vader ultimately overpowered and killed Momin by smashing him against the wall with one of the rectangular stones around the table. Having defeated all his enemies, Vader finally reopened the portal to resurrect Amidala.
While his physical body remained in the cave, Vader's consciousness passed through the gateway. Inside, Vader encountered distorted visions of various figures and events from his past, and he also glimpsed into the future, including his confrontation with his former Jedi Padawan, Ahsoka Tano, at the Sith temple on Malachor. Although he eventually reached a vision of his wife, he was unable to save her, as the apparition rejected him, claiming that Anakin Skywalker, Vader's former self, was dead.
Vader's vision was interrupted by the appearance of a distant figure wielding a blue lightsaber—Vader's son, Luke Skywalker, whose existence was unknown to the Sith Lord at the time. As Vader regained consciousness, he closed the gateway by stabbing the rock table with his lightsaber. The portal's destruction caused the ground in front of the castle to collapse, revealing the buried castle that had once belonged to Lady Corvax.
Around 3–4 ABY, a group of Force-sensitive warriors known as the Knights of Ren executed a heist on Vader's fortress to steal the Screaming Key for Lady Qi'ra, the leader of the Crimson Dawn criminal syndicate. To enter the castle undetected, the band's leader, "Ren," chose to use the old entrance to the Sith cave, which was unguarded and hidden behind the flowing lava. Ren combined his power with his companions—Bazzra, Cardo, Fyodor, Marinda, Massif, and Vicrul—and used the Force to bypass the lava flow. Upon entering the ancient cave, Marinda quickly sensed the location's strong connection to the dark side, which the Knights of Ren called the "shadow." Bazzra, on the other hand, noticed what appeared to be Momin's helmet on the floor, with the Sith Lord's severed head still inside. Provoked by the mask's apparent activation when she tried to reach it, Bazzra quickly drew her blaster and shattered the artifact. The group then continued into the castle, successfully stealing the key from Vader's vault.
Decades later, Snoke, the First Order's Supreme Leader and a Force-sensitive genetic strandcast secretly created by Sidious, wore a ring with obsidian carved from the Sith cave. The golden ring was also etched with glyphs of the Dwartii. Around the same time, Sidious mentioned the dark side locus on Mustafar while chronicling Sith history in his book, The Secrets of the Sith, stating that he believed the dark nexus still held great secrets despite Vader's failed attempt to resurrect his wife. Luke Skywalker, who had become the last Jedi Master, also mentioned the ancient cave in his book, The Secrets of the Jedi, where he chronicled the history of the Jedi Order.
The Sith cave was conceived as part of the backstory for Darth Vader's castle, which was designed for Rogue One: A Star Wars Story, the first film in the Star Wars Anthology Series. The cave was first mentioned in The Art of Rogue One: A Star Wars Story, published on December 16, 2016, the same day as the film's release. It first appeared in the comic book Darth Vader (2017) 5, written by Charles Soule with art by Giuseppe Camuncoli, and released by Marvel Comics on September 6, 2017. The final story arc of the Star Wars: Darth Vader (2017) comic-book series, Fortress Vader, further explored the creation of Vader's castle. The 2019 reference book Ultimate Star Wars, New Edition later identified the cave as a Sith shrine. Additionally, LEGO's Darth Vader's castle set includes an ancient Sith shrine with a Sith holocron beneath the castle, referencing the Sith cave.
According to Doug Chiang, co-production designer for Rogue One, the idea of a Sith cave beneath Darth Vader's castle originated from a concept art piece by Ralph McQuarrie depicting Emperor Palpatine's throne room for the 1983 film Star Wars: Episode VI Return of the Jedi. Chiang found the image of the throne on a lava lake inside a cave compelling and decided that the castle would be built on the foundation of an ancient structure. He wanted to give the castle's final design a sense of purpose, which was to draw energy from the lava lake like a dam. Furthermore, Chiang envisioned a more ancient part beneath the castle, in the form of a natural cave where Vader could meditate.
The 2019 reference book Star Wars: The Rise of Skywalker: The Visual Dictionary's timeline establishes that Darth Vader took up residence in "a newly constructed castle" on Mustafar twelve years before the first Death Star was ready. However, "Darth Vader," a 2020 entry in the Star Wars Encyclopedia series of reference booklets, states that the castle was built about five years before the completion of the Death Star. Since Star Wars: Timelines also supports the former's dating by placing the event in 12 BBY this article assumes "Darth Vader" is incorrect.
"Darth Maul and Other Followers of the Dark Side," a 2021 entry for Star Wars Encyclopedia, states that Vader returned the Mask of Lord Momin to Darth Sidious, who later stored the artifact aboard the yacht Imperialis until the vehicle's theft and subsequent destruction, following the events of the Fortress Vader story arc. However, the 2022 comic Crimson Reign 4, set after the aforementioned events, depicts the mask still lying inside the Sith cave, seemingly untouched since the resurrected Momin's death. According to Charles Soule, the writer of Crimson Reign 4, the mask seen inside the cave was merely a ghost.
Although the 2022 Star Wars Encyclopedia entry "Mustafar and Other Planets in the Outer Rim" accurately recounts the events related to Fortress Vader's construction over the Sith cave, it incorrectly states that Vader's castle was an ancient Sith temple that Vader later inhabited on one occasion.