During Nightswan's campaign, a clash of naval forces occurred in the skies above the planet Umbara. This conflict involved the Galactic Empire against those directed by the enigmatic Nightswan. This event, also referred to as the mission to Umbara, concluded with an Imperial triumph. Captain Thrawn played a key role by instructing Admiral Carlou Gendling's task force to target Nightswan's Vulture droids when they were within the signal shadows created by the Imperial ships.
During the Clone Wars, the Umbarans of the world Umbara chose to leave the Galactic Republic and join the Confederacy of Independent Systems following the death of Senator Mee Deechi at the hands of Lolo Purs. This secession led to an invasion by clone troopers to bring the planet back under Republic control. Despite some initial resistance during the early years of the Imperial Era, Umbara eventually became part of the Galactic Empire, though it maintained some level of self-governance. The planet would accumulate a significant debt to the Empire. Umbara was a significant source of doonium, a metal crucial for starship construction. The Imperial Navy and the secret Death Star project required vast quantities of this resource. Nevil Cygni, formerly a pirate and now an insurgent leader known as "Nightswan," was involved in smuggling doonium, tibanna, and iridium to fund his insurgency against the Galactic Empire.
Finding it difficult to corrupt the Umbaran authorities, Nightswan resolved to provoke an insurrection on Umbara. His goal was to make it easier for the Empire to take over the planet's mining and refining operations, as he believed bribing the Empire would be simpler than bribing the Umbarans. As part of his plan, Nightswan's agents set up a transmitter on Umbara's outermost moon. He also used his criminal connections to acquire 400 decommissioned Separatist vulture droids to participate in the revolt.
Admiral Carlou Gendling was given the task of suppressing the uprising, and an Imperial task force was sent to accomplish this. Gendling's fleet consisted of the Imperial Star Destroyer Foremost, two Raider-class corvettes, and the Imperial light cruiser Thunder Wasp, which was under the command of Commander Alfren Cheno. Captain Thrawn, a Chiss officer who had joined the Imperial Navy to secure an alliance for the Chiss Ascendancy, served as one of Cheno's subordinates. Admiral Gendling initially intended to use the fleet to intimidate the Umbarans into submission, but instead, he was met with an attack by the vulture droids.
The Imperial task force was quickly overwhelmed by the swarm of Vulture droids. Commander Cheno instructed the Thunder Wasp's gunners to focus on intercepting and destroying enemy fighters that were targeting the Foremost's dorsal surfaces. Ensign Eli Vanto, a friend of Thrawn, analyzed the attack patterns and traced the Vulture droids back to Umbara's outermost moon. Thrawn concluded that the Umbarans were not responsible for sending the Vulture droids, as they were waiting to engage the Imperial forces when they were in a weakened state. With the Thunder Wasp and other ships struggling against the agile Vulture droids, Thrawn and Vanto, with Commander Cheno's permission, took control of the chief sensor officer's Hammerly's station.
Thrawn and Vanto carefully examined the Vulture droid's attack patterns and determined that they were being controlled by a transmitter located on Umbara's outer moon. After pinpointing the transmitter's location, Thrawn persuaded Commander Cheno to inform Admiral Gendling of their discovery. However, Admiral Gendling, unwilling to take any risks, disregarded Cheno and Thrawn's information. Thrawn then instructed the communications officer to contact all ships and direct them to target the transmission shadows, where the vulture droids relied on their own programming. This strategy proved successful, destroying all the Vulture droids and shifting the battle's momentum in favor of the Empire.
The Umbarans surrendered without conditions after the destruction of the vulture droids. Stormtrooper squads were deployed from the Foremost to oversee the insurgents' surrender. The Imperials suffered the loss of one Raider-class corvette, while the remaining ships, including the Thunder Wasp and Foremost, sustained significant damage. Despite the Imperial victory, Admiral Gendling was enraged that Commander Cheno had seemingly undermined his authority. Before Gendling could confront Thrawn, Commander Cheno took responsibility to protect Thrawn.
Cheno told Thrawn that he saw potential in him and was willing to sacrifice his own career to protect his crew, even though Thrawn did not want Cheno to protect him. Following the battle, Vanto investigated the ground transmitter station on Umbara's outermost moon and discovered that it had been operated by humans. Vanto shared his findings with Thrawn, and they both concluded that Nightswan had orchestrated the Umbaran uprising to facilitate the Empire's takeover of Umbara's mining industry. Eli believed that Nightswan thought bribing Imperial inspectors would be easier than bribing the Umbarans. Thrawn saw the Umbara incident as a part of Nightswan's campaign against the Empire.
The unconditional surrender and Imperial victory in the engagement led to the Umbaran people surrendering their homeworld entirely to the Imperial regime, thereby giving up their remaining independence. By surrendering Umbara, the Umbarans were able to settle their debt to the Empire; however, there was no way for the Umbarans to settle that debt without giving over the last of their autonomy. As a result, Umbara came under direct Imperial control.
The battle over Umbara was first introduced in Timothy Zahn's 2017 novel Thrawn, a Canonical story that explores the origins of the character. The notion that Umbara had possessed a degree of independence under the Empire, which it was later forced to relinquish, was mentioned in the later 2017 novel Leia, Princess of Alderaan. However, the connection between this surrender and the battle depicted in Thrawn was not established until the publication of the 2019 reference book Ultimate Star Wars, New Edition.