During the Imperial Era, a garrison was stationed on Mustafar as part of the security for Fortress Vader. During the castle's construction, the garrison was commanded by Captain Junus, and participated in the Battle of Fortress Vader, where it was annihilated. Shortly afterwards, the garrison was supplemented to battalion-level, where it continued to provide security for Fortress Vader and a nearby Imperial base.
The garrison that was ordered to protect Fortress Vader's construction on the Gahenn Plains on Mustafar first consisted of many resources of the Galactic Empire. Imperial construction crews built the many designs of the fortress, while they were defended by Imperial magma troopers. Imperial officers were part of the garrison as well, and the garrison deployed AT-ACTs, AT-STs, and TIE fighters for vehicle reinforcement. An Imperial-class Star Destroyer was also stationed above the fortress in Mustafar's atmosphere at one point during the construction. At this time during the castle's construction, the garrison's commander was Captain Junus.
After the garrison was wiped out during the Battle of Fortress Vader, it was rebuilt to continue to provide security for the fortress. Based on the far side of the plains, the castle's security consisted of lava troopers and security droids, as well as Imperial Royal Guards. AT-DPs were added to the security's arsenal.
By the time of the mission to Mustafar between 2 BBY and 0 BBY, an Imperial base with additional security was built near the castle on the lava lake, and supplemented by standard stormtroopers. Around this time, Admiral Gable Karius was the leader of the Imperial garrison, and were considered a battalion. In addition to this growth, Karius had under his command a Star Destroyer-sized Interdictor-class, and standard stormtroopers provided security for Fortress Vader as well. Imperial forces were also on Lady Corvax's sanctum located beneath Fortress Vader on occasion.
During the Imperial Era, Darth Vader traveled to Mustafar to create his own stronghold there. After the Mask of Lord Momin activated and revealed itself to Vader, construction began on the castle. During construction, a garrison was created to provide security for Fortress Vader and construction crews. During this time, the garrison defended the construction site from lava fleas, which attacked more frequently as each design of the fortress was activated and subsequently destroyed. It wasn't until the ninth design of Fortress Vader the the structure maintained its integrity when activating the locus of the Force found in the Sith cave, located beneath the fortress. While Vader activated the fortress, the garrison and castle came under attack by a Mustafarian army led by Father Kkkt, culminating in the Battle of Fortress Vader.
Although Captain Junus recommended that the garrison evacuate, Vader joined the frontlines and belayed his orders, instead leading the garrison against the attackers while Sith Lord Darth Momin was resurrected from the past by his mask when he activated the fortress. The entire garrison's ground forces were wiped out in a wave of lava created by Father Kkkt, although Vader managed to do the same to the Mustafarians when he re-entered his fortress and activated it once more.
After the Battle of Fortress Vader, the garrison was reformed to continue to provide security for the fortress. Soon after the completion of construction, the castle's security was involved in a raid on the castle, when a group of Southern Mustafarians led by Criakan managed to break into the castle, although they were all defeated by Vader. The fortress continued to succumb to raids by Mustafarians during its time.
Later on, Mustafarians once again attacked Fortress Vader, this time albeit by Mother Sssl and a mind-controlling plague. Mother Sssl managed to attack lava troopers on patrol and have them succumb to the plague. Once again, however, Darth Vader defeated the Mustafarians and slaughtered them all.
- Ultimate Star Wars, New Edition
- Star Wars: The Rise of Skywalker: The Visual Dictionary