Originating from the Kilji homeland within the Unknown Regions, the Kiljis were a sentient species. Their distinctive appearance included dark orange skin, which was wrinkly and had a rubbery texture. The patterns formed by the stretching of their skin mirrored the emotions felt by individual Kiljis. Profoundly religious, the Kiljis were driven by a desire to spread the teachings of the Kilji religion throughout the universe, displaying hostility towards those who did not embrace their philosophical beliefs. Their nation was known as the Kilji Illumine, governed by the Generalirius Nakirre. The Illumine Kilhorde, the Illumine's formidable battle fleet, embarked on conquests of enlightenment across numerous worlds in the Unknown Regions.
In the year 18 BBY, Nakirre formed an alliance with the Grysk operative known as "Jixtus." Jixtus pledged to oversee the Kilhorde's conquests, and in exchange, Nakirre granted Jixtus passage aboard his Kilji war cruiser, the Whetstone. Their joint objective was to undermine the Chiss Ascendancy. Their plan involved triggering a civil war within the Ascendancy by manipulating the influential Chiss families into engaging in internal conflict. Meanwhile, General Crofyp's task force, which included the Kilji war cruiser Anvil and Colonel Tildnis's Kilji picket cruiser Hammer, launched attacks on several worlds in the Unknown Regions. However, the Senior Captain of the Chiss, Mitth'raw'nuruodo, destroyed the Anvil and retrieved crucial data regarding the Kilhorde from the Hammer. This information was then shared with the Paataatus Hiveborn, leading to the obliteration of the Kilji battle fleet. Consequently, Jixtus deemed Nakirre expendable and murdered him.
The Kiljis were a sentient species characterized by dark brown hair and distinctive skin. Their skin had a rubbery, wrinkly texture and was dark orange in color. The patterns formed on their skin mirrored their emotional states. As bipedal beings, Kiljis possessed voices that were raspy and lacked the melodious quality found in members of the Grysk species.
The Kiljis possessed a profound religious devotion, leading them to conquer worlds and subjugate species. Their ultimate aim was to enlighten these populations with the tenets of the Kilji religion, known as the Kilji way of order and enlightenment. Those who embraced the Kilji way became Kilji vassals, willingly surrendering their freedom in exchange for the Kiljis' care. These vassals were subservient to their Kilji commanders, lacking independent thought and action without the guidance of their leaders. Orders from those not enlightened to the Kilji philosophy were disregarded by the vassals. The Kilji religion emphasized unwavering commitment to battle and the pursuit of victory. Followers of the Kilji way displayed hostility towards the unenlightened, characterized by impatience and a quick temper.
The warships of the Kilji nation, the Kilji Illumine, were engineered with a focus on initiating swift and decisive strikes. The Kiljis harbored a desire to spread the teachings of the Kilji way to the universe's inhabitants. They communicated using various languages, including the Kilji language and the trade languages of Minnisiat and Taarja. Kiljis employed multiscreen devices to display recordings across multiple screens with limited magnification. Their starships incorporated peace-sealing plating to conceal their weaponry.

The Kilji homeland, located in the Unknown Regions, served as the Kiljis' point of origin. Ancient Kilji doctrines asserted that embracing Kilji enlightenment was rarely an individual's initial choice and that such a choice could be subject to change. Furthermore, the doctrines declared that the fate of those who opposed the Kilji way was unalterable. In 18 BBY, the Illumine was under the leadership of Nakirre, a Kilji generalirius whose decisions were subject to the approval of the Kilji Overlords. Nakirre held command over the Illumine Kilhorde, the Illumine's battle fleet, as well as the Kilji war cruiser Whetstone. The generalirius entered into an alliance with the coordinator of the Grysks, a Grysk operative known as "Jixtus," who pledged to guide the Illumine's conquests. In exchange, Nakirre granted the operative permission to travel aboard the Whetstone while they jointly sought to conquer various nations within the Unknown Regions.
The pair journeyed to Zyzek, a planet in the Unknown Regions, where they encountered the Chiss heavy cruiser Springhawk and its commander, Mitth'raw'nuruodo, a Chiss Senior Captain whose core name was "Thrawn." The Chiss transmitted a message claiming to hold Watith prisoners, prompting Nakirre to offer assistance in returning the Watith to their home, citing the Kilji religion's mandate to serve others. However, Thrawn declined the offer, leading to a brief confrontation between the generalirius and the Chiss, which was ultimately resolved by the Zyzek System Defense. Subsequently, the Kilji war cruiser departed Zyzek, heading towards the Chiss Ascendancy, the Chiss species' nation. Jixtus sought to destabilize the Ascendancy by manipulating its families into engaging in internal warfare.

Nakirre and his ally visited the Chiss planets of Avidich and Rhigar, where they engaged with Thistrian, the Patriel of the Mitth family, and Clarr'os'culry ("Roscu"), a Captain, respectively. Roscu boarded the Whetstone, and the Kilji generalirius extended an offer to provide the Clarr with several Kilji warships, an offer that the captain declined. While the Kilji and the Grysk worked to sow discord among the Ascendancy's families, Jixtus dispatched General Crofyp's task force, consisting of Crofyp's war cruiser Anvil and Colonel Tildnis's Kilji picket cruiser Hammer, to the Rapacc system in the Unknown Regions. The Kilji force attempted to seize a group of refugees who had sought refuge on the planet Rapacc. However, the task force was confronted by the Nikardun blockade frigate Aelos, which had been commandeered by the Paccosh.
After Crofyp ordered his vessels to prepare a cross-fire attack on the Aelos, the Springhawk arrived in the outer system and disabled the Anvil's starboard side using plasma sphere weaponry. The Chiss destroyed Crofyp's vessel, while the Hammer managed to escape to the "Sunrise" system in the Unknown Regions after being shielded from the Chiss attack by the larger Kilji vessel. The crew of the Aelos and Thrawn tracked the picket cruiser to the Sunrise system, where it was stationed alongside one of Jixtus's Grysk warships. A squad of Paccian commandos and the senior captain boarded the Hammer, eliminating all the Kiljis on board, including Tildnis. Thrawn instructed Sarsh, a Pathfinder navigator, to retrieve a Kilji data rectangle containing information about the Kilhorde before departing the Kilji picket cruiser. Jixtus's vessel subsequently destroyed the Kilji picket cruiser before fleeing into hyperspace.

As the Ascendancy's families engaged in battle and Nakirre and Jixtus met with various Chiss officials, conflicts arose between the Kilji and Jixtus. While the Ascendancy was being destabilized as Jixtus intended, Jixtus mocked the generalirius and disparaged the Kilji path, characterizing it as simplistic, self-contradictory, and self-defeating. This mockery of the Kilji way enraged Nakirre, who considered murdering Jixtus but ultimately decided against it, knowing that such an act would provoke the Grysk fleet to attack the Kilji homeland and eradicate the Kilji species. Consequently, the Kilji followed the Grysk coordinator's every command. Jixtus continued to taunt the Illumine, threatening to crush it and distort the Kiljis' culture, souls, and lives, forcing them to serve the Grysks without question. After the Grysk granted the Kilhorde permission to advance against the nations southeast of the Chiss nation, the battle fleet conquered several worlds and species, aiming to enlighten them with the Kilji way.
However, Thrawn shared the data he had obtained from the Hammer with the Paataatus Hiveborn, who deemed the Kilji force's operations near its northeastern borders unacceptable. The Chiss senior captain highlighted the weaknesses in the Kilhorde's design and battle doctrine, leading the Paataatus attack fleet to shatter the battle fleet and liberate the conquered worlds and peoples. After Nakirre received no reports of victory from his field commanders, he feared that his generals had extended their conquests to Colonial Station Chaf, a Chiss world, an attack that would disrupt the operation of the world's long-range triad transmitter. He speculated that the lack of communication from his commanders was due to the disruption of the transmitter's operation, which served as the sole link between them and their ruler.
Shortly thereafter, Thrawn assembled a substantial fleet of Chiss Expansionary Defense Fleet vessels and brought it to Sunrise. He and his Expansionary Defense Fleet colleagues deactivated the fleet and scattered debris throughout the system, preparing for Jixtus's arrival. When the Whetstone and Jixtus's fleet arrived, the Chiss senior captain informed Nakirre that the Hiveborn had destroyed his battle fleet. In response, the generalirius lashed out at his Grysk ally, asserting that the Kilhorde's destruction by the Paataatus was due to the coordinator's inadequate support for the Kilji conquests of enlightenment and his failure to accept the Kiljis as true allies. Jixtus retaliated by murdering the generalirius with a blaster, causing Nakirre's Kilji vassals to enter a catatonic state and mourn their deceased master.
As Thrawn's fleet reactivated and began to decimate Jixtus's fleet, the Grysk fled to his Shatter-class WarMaster FateSpinner flagship, leaving Qilori, the Whetstone's Pathfinder navigator, alone with Nakirre's vassals. After his fleet had been reduced to seven vessels by Thrawn's forces and vessels from the Garwian Unity, Paccian Governance, and Vak Combine nations, Jixtus prepared to escape. Qilori realized that the Grysk might destroy the Whetstone during his escape, killing him in the process to eliminate any non-Chiss witnesses to the battle. He promised to lead the Kilji vassals until he could return them to their home after one of them revealed their inability to think for themselves. Under the Pathfinder's direction, the vassals unleashed all of the Whetstone's weaponry on the FateSpinner, destroying its aft flank and prompting Jixtus to activate a self-destruct mechanism aboard his flagship, killing himself. Shortly after, Thrawn instructed Qilori to guide the Kiljis back to their home.
The crews of Crofyp's Anvil and Nakirre's Whetstone were composed of numerous Kilji vassals. The former war cruiser's crew included Vassal One, a weapons operator; Vassal Two, a sensor operator; Vassal Three, another vassal who operated the Anvil's weaponry; and Vassal Four, the cruiser's comm officer. Nakirre's vassals included Vassal One, a pilot; Vassal Two, a sensors operator; Vassal Four, a comm operator; a helm vassal, who informed Qilori about his and his fellow vassals' inability to think for themselves; another comm vassal; another sensors operator; another pilot; and a weapons vassal. Tildnis's Hammer carried seventeen Kiljis.
The Kilji species made its debut in the 2021 novel Thrawn Ascendancy: Lesser Evil, which served as the concluding installment in the Star Wars: Thrawn Ascendancy Trilogy penned by Timothy Zahn.