The attack on Csilla was a battle between the forces of the Chiss Ascendancy and three attacking warships. The attackers were the Paataatus Hiveborn, though they were unidentified at the time of the attack. The Hiveborn had been ordered to attack the Chiss homeworld and capital of Csilla by their conquerers, the Nikardun Destiny, to distract from a Nikardun massacre of refugees at the Dioya system at the Ascendancy's edge. In turn, the Nikardun had conquered the Paataatus and other nations near the Ascendancy at the behest of the Grysk Hegemony, who sought to encircle, undermine, and ultimately invade the Chiss Ascendancy.
The attackers were quickly defeated by the Chiss Defense Force. However, the attack had occurred during a legislative session of the Chiss Syndicure, sending the Ascendancy's ruling Aristocra into an uproar. The Syndics became single-minded about identifying and punishing the attackers, as no-one had dared to attack Csilla in decades. Supreme General Ba'kif of the Chiss Expansionary Defense Fleet thought the question of why Csilla had been attacked was of more interest than who had attacked it. Nevertheless, Ba'kif consented to a retaliatory strike on the Paataatus homeworld to punish the Paataatus for proven raids they had recently conducted against the Ascendancy.
The Grysk Hegemony, an expansionist empire led by the warlike Grysk species, sought to conquer the Unknown Regions, known to locals as "the Chaos." The Grysk agent "Jixtus" had groomed the Nikardun general Yiv "the benevolent" to lead the Nikardun Destiny in conquering the nations nearby and surrounding the Chiss Ascendancy. Jixtus plotted the Ascendancy's destruction. Ascendancy military doctrine forbade military action against forces who had not previously attacked the Chiss, even if they posed a clear threat. As such, a nation could conquer the Ascendancy's neighbors to prepare to invade the Chiss without legal reprisals from the Chiss Expansionary Defense Fleet. Jixtus viewed the Nikardun encirclement of Chiss territory as the first phase of a successful invasion.
Among the nations near the Ascendancy were the Paataatus, a species whose government had often raided Chiss border systems. By around 19 BBY, Jixtus guided Yiv to conquer the Paataatus. Meanwhile, Jixtus had sent agents of the telepathic Agbui species to engineer a civil war on the planet Sunrise. The ensuing civil war decimated the native population, and the survivors were subsequently enslaved by the Grysks to mine nyix, a valuable metal used to reinforce the hulls of warships. However, a group of refugees led by the Magys escaped Sunrise to the Rapacc system, where they established a colony among the native Paccosh. Haplif, leader of Jixtus's Agbui saboteurs, contacted Nikardun forces to eliminate the Magys' refugees. As a result, the Nikardun conquered and blockaded Rapacc. However, the native Paccosh had managed to send two refugee starships towards the Chiss Ascendancy, whom they intended to plead for help against the Nikardun.
Around 19 BBY, one of those two Paccian refugee ships reached the Ascendancy's peripheral Dioya system, where the Nikardun massacred them. Yiv ordered the Paataatus to attack the Ascendancy's capital planet of Csilla to distract the Chiss from the massacre.
Three Paataatus warships appeared above the Chiss homeworld and capital of Csilla from widely spaced hyperspace vectors and promptly attacked the Chiss defense platforms above the planet with their spectrum lasers. Within one minute, the Chiss Defense Force began returning fire. The attacking warships bombarded the Chiss capital city of Csaplar with missiles, but the defense platforms managed to intercept the missiles from orbit so they caused minimal damage on the surface. The Chiss warships were successfully able to destroy the invading warships within fifteen minutes of the attack's commencement.
The attack took place on a day when the Chiss Syndicure was in full session. As a result, most of the Ascendancy's top-level Aristocra were gathered at Csaplar's Assembly Cupola for high-level government business. The attack sufficiently frightened the Syndicure to request reinforcements from the Chiss Expansionary Defense Fleet at Csilla and other inner Chiss holdings, at the cost of recalling them from the border regions of the Ascendancy. Indeed, the reduction of forces at the periphery of the Ascendancy was a goal of the unknown enemy behind the attack.
The enemy behind the attack was really General Yiv and his Nikardun Destiny, who had launched the attack to cover up the attack on the edge of the Dioya system. This distraction was realized by Supreme General Ba'kif, though the Ascendancy blamed the Paataatus government, the force sent by the Destiny so they did not need to use their own craft. This led to a Chiss retaliation in the form of the attack on the Paataatus, but Thrawn continued his investigation into the true masterminds, leading to his discovery that the Nikardun were the true threat.
After the battle, Supreme General Ba'kif sent Senior Captain Mitth'raw'nuruodo "Thrawn," who was under treatment for injuries sustained in the Vagaari pirate operations at a medcenter on Naporar, to investigate what event might have happened that the attack on Csilla was meant to distract the Ascendancy from discovering.
From Thrawn's analysis of recordings of the attack, he surmised that the warships behind the attack were old and even "slightly obsolete." Therefore, Thrawn told Ba'kif he thought the ships would not resemble the current ships being used by the enemy behind the attack.
The attack on Csilla serves as the prologue to the 2020 novel Thrawn Ascendancy: Chaos Rising, the first installment of Timothy Zahn's Ascendancy Trilogy. The investigation of the attack sets General Ba'kif and Senior Captain Thrawn up to discover the threats that serve as the main conflict in the novel.
- Star Wars: Timelines