Lava fumes were toxic gases emitted from lava lakes located deep underground on the planet Mustafar. When inhaled, the fumes played tricks on the mind, giving rise to voices and visions of fears and doubts that could manifest as ghosts of the past. Sustained exposure to the gases drove an organic sentient insane, and repulsor vents were thus installed at indoor facilities to prevent the fumes from being inhaled by those within. Lava-fumes drifted upwards to surface-level facilities, and were most potent around the vicinity of Fortress Vader and the Klegger Corp Mining Facility.
After the death of Darth Vader, the mustafarians Tuttel and Giggek attempted to destroy Fortress Vader. They successfully damaged the castle, and upon exiting the castle, the lava fumes caused Vader's servant Vaneé to think that he was hearing the voice of his dead master. The voice of Vader caused by the lava fumes informed Vaneé that in order to revive his master, he would need to preform a ritual to capture Milo Graf by giving him a nightmare of zombie droids attacking Anakin Skywalker, Padmé Amidala, C-3PO, R2-D2, and Jar Jar Binks on during the Clone Wars. The Sith ritual was then able to possess Graf's droid CR-8R and bring him to Mustafar.
Vaneé and the lava fumes then began to use nightmares to lure other people to Mustafar, giving one to the smuggler Jaxxon T. Tumperakki where he encounters the wookiee Chewbacca turning into a giant monster. Milo's sister Lina Graf, who realized that her brother Milo was gone, recruited Jaxxon to travel to Mustafar and uncover the source of the nightmares. The Rebel Thom Hudd also had a nightmare caused by the lava fume ritual where he and Luke Skywalker had to fight the "Spirit of the Swamp," he and Skritt then chose to join Lina in uncovering the source of these visions. On the way to Mustafar, Lina Graf fell asleep and also had a nightmare caused by Vaneé's ritual, where she encountered Anakin Skywalker at various ages in his life. In the nightmare, Lina encounters CR-8R's mangled body, and Milo becoming a revived version of Vader. She believes she's woken up from the nightmare only for a revived Vader to attack her, causing her to truly wake up. Upon landing, she comments how the dreams were getting worse.
Upon arriving, Vaneé captures the Rebels and reveals that he was using the lava fumes to control their nightmares. He attempted to use the vapors to revive Vader, causing the group to view all of their fears. Vaneé believes he has succeeded in possessing Lina's body with Vader's spirit, only for her to reveal that Vaneé only thought he heard Vader, as he was what Vaneé feared most. The rebels trapped Vaneé in with the fumes to endure a lifetime of suffering.