Nacronis, situated in both the Mid Rim and New Territories regions, was a planet characterized by marshland terrain. Its unusual weather patterns led to frequent and devastating siltstorms. Humans and other beings populated this world, dwelling within age-old strongholds. In the year 4 ABY, as part of the infamous Operation: Cinder, Galactic Empire forces, specifically the TIE fighter pilots belonging to the 204th Imperial Fighter Wing, received directives to amplify the planet's storm intensity through the use of explosives. This attack resulted in widespread destruction and the deaths of the planet's entire population.
Following this act of genocide, Major Soran Keize from the 204th persuaded Imperial pilot Yrica Quell to abandon her allegiance and desert the Empire. Quell then aligned herself with the rising New Republic, presenting a somewhat fabricated account of her reasons for switching sides. Although the truth regarding the Nacronis tragedy eventually came to light, Quell avoided prosecution after the fall of the Empire and eventually encountered a survivor who had lived through the genocide.
Nacronis was a terrestrial planet within the Nacronis system, located at the intersection of the Mid Rim and New Territories regions, specifically at grid square M-7 on the Standard Galactic Grid. Its Type I atmosphere was breathable for humans. The planet's unique meteorology was defined by powerful winds and siltstorms that flung colorful mud into the air. This phenomenon transformed the surface into a vast marshland. The severity of these siltstorms was deliberately increased during the Galactic Empire's devastation of the world.
During the Galactic Civil War between the Galactic Empire and the Rebel Alliance, the supposed death of Emperor Palpatine during the Battle of Endor in 4 ABY triggered Operation: Cinder, an Imperial initiative to obliterate planets regardless of their allegiance. That year, Nacronis was selected as a target for this operation, despite not posing any threat to the Empire. In fact, none of the Imperials assigned to its destruction had even heard of Nacronis prior to the mission. Colonel Shakara Nuress of the Empire's 204th Imperial Fighter Wing was ordered to take part in the destruction, finding the operation distasteful but not questioning the order.
Alongside other Imperial squadrons, the 204th's pilots, under the command of Major Soran Keize, were tasked with intensifying Nacronis's siltstorms using specialized vortex detonators deployed from their TIE bombers, as the satellites used elsewhere were unavailable. Despite the Rebellion's attempts to defend Nacronis, their forces were unable to stop the Imperial pilots from amplifying the storms to encompass the entire planet. The resulting devastation destroyed cities in floods of silt, wiping out the entire population and all life on Nacronis's surface.
After the storms subsided, Yrica Quell, one of the 204th's pilots, landed on Nacronis and witnessed the destruction she had helped to create. Major Keize, who had also lost faith in Operation: Cinder, convinced Quell to defect by staging her own death. Keize also expressed his feelings to the squadron commanders before abandoning the 204th himself.
Approximately one month following the attack on Nacronis, Quell was recruited into the newly formed New Republic, the successor to the Rebel Alliance, as part of Alphabet Squadron, whose mission was to hunt down members of Quell's former Imperial fighter wing. By this time, the destruction of Nacronis was well known within the New Republic, which regarded it as one of the Empire's most heinous massacres. Quell initially concealed her involvement in the atrocity, but New Republic Intelligence agent Caern Adan ultimately uncovered the truth, as did fellow pilot Nath Tensent.
Adan programmed an automated message containing the truth about Nacronis to be delivered to New Republic personnel in the event of his disappearance. This message was triggered when he was abducted during the New Republic's campaign in the Deep Core's Cerberon system in 5 ABY. Subsequently, Quell experienced numerous mental visions of the destruction of Nacronis, induced by a tower situated on a planetoid within the Cerberon system.
After her actions were revealed, Quell rejoined the 204th with the intention of bringing about the fighter wing's downfall. During her time with her former unit, which had since embarked on a second destructive Cinder campaign, Quell experienced vivid mental images of the genocide on Nacronis and other worlds it had attacked. Upon returning to the New Republic, Quell confessed the complete truth about her defection on Nacronis to General Hera Syndulla. During a confrontation with Keize on the Core Worlds capital planet Coruscant, Quell's former mentor exonerated her from any direct responsibility for the genocide, but this did not alleviate her guilt. Keize was killed while trying to destroy a data bank that documented Imperial crimes and atrocities, while the 204th and the Empire itself were defeated by the New Republic at the Battle of Jakku.
With the war concluded, the New Republic decided to allow Quell to remain free, despite her involvement in the destruction of Nacronis. Six years afterward, one survivor of the Nacronis genocide, who had been off-world at the time but had lost family members who remained, contacted the New Republic's Reconciliation Project, expressing a desire to speak with uncooperative former Imperial pilots. Quell agreed to Senator Wyl Lark's request that she meet with the survivor before visiting the imprisoned.
Eventually, a file detailing the 204th's actions on Nacronis was included within a file labeled "Nacronis" in the Operation Cinder section of the Rebel Alliance Archives. In 35 ABY, historian Beaumont Kin wrote and published a book titled The Rise and Fall of the Galactic Empire, documenting the Empire's rise and fall. In the book's twenty-first chapter, "Fracture and Cinder," Kin recalled the world's destruction during Operation Cinder, noting it was one such planet that had not been as fortunate as the Mid Rim world Naboo which had been saved from destruction.
The population of Nacronis, consisting of several million humans, was eradicated during Operation: Cinder. This act of genocide eliminated the planet's civilization, although one resident was off-world at the time. "Silt Sea Threnody" was a burial song originating from Nacronis. When providing a false account of the events that led to her defection, Yrica Quell claimed to have contacted Nacronis ground control.
The inhabitants of Nacronis resided in ancient citadels that were destroyed during the Empire's devastation of the planet. These cities, comprised of domed structures built from duracrete, included apartments, houses, schools, and workshops.
Nacronis made its first appearance in flashbacks within the novel Alphabet Squadron, written by Alexander Freed as the first book in the Star Wars: Alphabet Squadron trilogy, published on June 11, 2019. Prior to the novel's release, Nacronis's appearance was featured in an excerpt printed in the first issue of the 2019 comic series Star Wars: TIE Fighter, published by Marvel Comics on April 17, 2019.