The Battle of Fortress Vader transpired on the planet of Mustafar at some point during the Galactic Empire's era. It was initiated when a Mustafarian army, under the leadership of Father Kkkt, revolted to protect their dwindling homeworld. This uprising was triggered by Darth Vader's construction of his castle, Fortress Vader, which caused immense suffering to the planet. Although Vader presented a significant challenge, Kkkt and the other chiefs harnessed the Force to inundate the battlefield with lava.
The entire Fortress Vader Imperial garrison was obliterated, with Vader as the sole survivor. He retreated into the fortress while the remaining Mustafarians attempted its destruction. Vader then utilized the dark side energies concentrated beneath the fortress to eradicate the Mustafarian army. Following this, the Sith Lord Darth Momin confronted Vader in the focusing chamber, seeking to kill him. A battle ensued, during which Momin severely wounded Vader, only to be crushed by a falling stone. Now alone, Vader opened the gateway to the dark side of the Force and stepped through.
After killing two renegade Inquisitors during a pursuit on Coruscant, Emperor Palpatine gifted Darth Vader the planet Mustafar as the location for his personal bastion. The stronghold was conceived by the incorporeal essence of an ancient Sith Lord known as Darth Momin. The construction process unleashed a substantial amount of dark side energy, which was detected by a Force-sensitive elder from a nearby clan of Mustafarians. Through numerous construction iterations, Darth Vader and Momin attempted to create a dark side portal. Each instance the portal began to open, it triggered catastrophic consequences on the surrounding environment, posing a grave threat to the native tribes and wildlife. Minor Mustafarian rebellions arose in response to the castle's construction. On occasion, large swarms of lava fleas assaulted the castle, even surprising Momin. However, these small uprisings were easily suppressed by the Imperial garrison.
Upon completion of the ninth version of Vader's castle, the Sith once more attempted to open the portal. However, they were interrupted by reports of an unprecedented attack by the native population, led by Father Kkkt. Annoyed, Vader was compelled to assist his forces in repelling the attack to prevent the castle from being overrun and destroyed. In Vader's absence, Momin reactivated the portal, despite his claims of being unable to wield the Force in his disembodied form. Momin's original body emerged through the portal, fully resurrecting the ancient Sith after thousands of years.
The Imperial forces were quickly overwhelmed by the attackers and attempted to retreat. However, Vader managed to reverse the course of the battle and rally his troops. Seeing an opportunity to eliminate Vader, Kkkt gathered more mystics from his tribe to invoke the "Blood of Mustafar." Kkkt and his mystics used the Force to summon a massive wave of lava that engulfed all of the Empire's forces, leaving only Vader alive. Vader used the Force to levitate himself to safety, but suffered significant damage to his armor and cybernetics. Vader reentered the cavern beneath his castle to open the portal, which once again unleashed catastrophic levels of dark side energy, incinerating the remnants of the Mustafarian resistance.
Upon Vader's return to the cave, Momin launched an attack, and the two Sith Lords engaged in a lightsaber duel. During the fight, Momin mocked Vader, asserting that he would never succeed in opening the dark side door, as it would reject him. Momin lamented the perceived decline of the Sith's power since his death, labeling Vader and Sidious "Jedi-obsessed weaklings." Vader sustained severe injuries during the duel, but as the ancient Sith Lord proclaimed the dark side's love for him, Vader used the Force to crush Momin beneath a massive boulder. A disbelieving Momin met his demise for the second time.
With Momin eliminated, Vader personally opened the gateway to the dark side, and his spirit departed his body to enter it. On the other side, Vader witnessed a series of visions representing his life before reaching an illusion of the Jedi Temple. There, he confronted and slaughtered illusions of Jedi he had encountered in the past. After killing an illusion of Palpatine, Vader encountered his beloved, Padmé Amidala. He intended to revive her, but she instead jumped from the balcony and was destroyed. Devastated, Vader was soon distracted by a beam of light. The being within overwhelmed him, and Vader's spirit returned to his body. Having failed to resurrect his wife, Vader destroyed the portal and contacted Palpatine. Later, he returned Momin's mask, still containing the essence of the rogue Sith Lord, to Palpatine.