The Battle of Achiliquin was a major battle between the New Republic and the Empire, fought during the third year of the Second Great Cyrannus War. The sole beacon of Republic rule in the Inner Rim, the Empire sought to capture the famously tranquil world, and thus deal a crippling blow to the Republic's morale, which had surged following the Empire's embarrassing defeat at the Battle of Cyranduas.
With the bulk of the Republic's meagre Inner Rim defence fleet redeployed to protect the Capricorn Sector, the Phaedric Lord Maethoruin and Admiral Gaius Prentus arrived in the system and bombarded the planet from orbit, wiping away all naval opposition with the power of the Executor-class Star Dreadnought Dark Hand. Seeking to cripple the Republic's response planetside, Maethoruin bombarded several major population centres, and captured the planet's government as well as Senator Amirtae Buroa. Coordinating with the Aldárae Order, the Republic diverted a response fleet to nearby Vondora, a single jump-point away from Achiliquin, and deployed teams of Aldárae warriors under the direct command of Grandmaster Ryen, as well as Banshee Squad under Commander Corvexa, to coordinate with local security forces.
Republic forces engaged in highly successful guerrilla tactics against the Empire, resulting in the eventual release of the Achiliquini Council, and culminating in the Battle of Whithil Mountain, during which Grandmaster Ryen and his forces led a surprise attack against Lord Maethoruin as he neared the capital Achili. A vicious, though inconclusive, duel erupted between the two, wounding both masters considerably. However, Ryen's forces, led in his absence by his tirolenros Arasah Nui, triumphed over the Imperial army, forcing it to retreat. With Imperial forces in disarray, the Republic armada, commanded by Fleet Admiral Gialu Kaedar dropped out of lightspeed, surrounding and destroying the Dark Hand, forcing Admiral Prentus to withdraw from the system. The Battle of Achiliquin sent shockwaves throughout the Empire, and emboldened the Republic to intensify efforts to push into Imperial-held territory.
The Battle of Cyranduas proved to be an unacceptable embarrassment to the Empire, which had hoped for a swift victory against the Republic following the fall of both Rambo Nation and the Allied Terran Republic at the close of 16 NE. While the battle resulted in the destruction of the Republic's army on Cyranduas, the planet was quickly reinforced by a Republic fleet following Admiral Irenwen's withdrawal, thus securing a symbolic Republic victory. While Grand Mandator Deoclet Caesarius began preparing for a new offensive against the Republic's Core Worlds, the Phaedric Lord Maethoruin began preparing for new offensive to take Coru Secundus, beginning with the Republic's sole bastion in the Inner Rim—Achiliquin.
Amassing a fleet in the Ord Bilbrin system, Maethoruin rallied a large fleet from his personal Executor-class dreadnought Dark Hand, while placing the bulk of his forces under the command of the newly promoted Admiral Gaius Prentus of the Star Destroyer Accuser. Drawing Republic forces away from Achiliquin with a feint attack on Eire in the Outer Rim, Maethoruin's armada arrived in the Achiliquin and quickly annihilated orbital defences, and any remaining Republic naval units. Lacking the planetary shield found on Cyranduas, Achiliquin soon faced the brunt of the Empire's fury, wit the Dark Hand raising several cities on Achiliquin's northern continent, creating firestorms which killed over three million people. Maethoruin soon began landing his forces, seizing Achiliquin's council as it fled into hiding, and capturing Senator Amirtae Buroa, who managed to send a distress beacon to Capricaerón moments prior to her capture.
When news reached the Republic, President Apollo quickly ordered Fleet Admiral Gialu Kaedar to drive off the Imperials and secure Achiliquin from invasion. Coordinating closely with Grandmaster Ryen of the Aldárae Order, Admiral Kaedar dropped out of hyperspace in the nearby Vondora system, a single jump-point away from Achiliquin, and from there launched several teams of Aldárae and Republic Special Forces, including Banshee Squad under Commander Corvexa. Landing in the Whithil Mountains outside the planetary capital Achili, the Aldárae, led by Ryen himself, began leading hit and run operations against the Imperial armies, robbing Maethoruin of the quick victory he sought. Nevertheless, while Fleet Admiral Kaedar hoped that the Aldárae would be enough to end the invasion immediately, their numbers were too few to make a dent in Imperial operations.
During the third week of intense guerrilla warfare across the planet, the Aldárae tirolenros Arasah Nui, sneaking behind enemy lines, ascertained the location of Senator Buroa, and led a mission along with Commander Corvexa to rescue her. Nevertheless, though Buroa announced to the planet via HoloDomain that the battle was not yet over, it was widely accepted that the planet had already fallen. The next day however, Ryen sprung his trap. As Maethoruin made his final push toward Achili, the combined Aldárae and Republic force ambushed the Imperial army—thus sparking the deciding Battle of Whithill Mountain. Though Commander Corvexa's forces utilised their knowledge of the terrain to their advantage, felling several dozen Imperial soldiers and wardroids, they were outmatched by Maethoruin himself, who single-handedly killed over a hundred foes—Republic and Aldárae, alike.
Stepping onto the battlefield in his old robes, Ryen calmly approached his foe, and according to accounts from the battle by surviving Republic troopers, commanded Maethoruin to leave the planet at once. The Phaedric Lord's reply came swiftly and furiously, rejecting the Osteola's overtures and engaging him in a ferocious duel. As Light and Dark clashed on the planet, Commander Corvexa's forces—joined by the remnants of the Achiliquini defence force, began pushing the Imperials back to their landing zone. While Maethoruin possessed the ferocity and physical strength to keep Ryen at bay, he could not match Ryen's mastery over the Light. After over an hour of constant skirmishing in the woods, during which both masters suffered grievous wounds, Ryen telekinetically blasted Maethoruin into the rocks, incapacitating the mighty Phaedric Lord, before the old Aldárae master collapsed from his own injuries.
In the chaos that followed, Arasah Nui rescued Ryen and returned him to their ship, while Imperial forces, fleeing in shambles with the fall of their commander, retrieved him and retreated into orbit. There, the situation had not improved. Capitalising on the chaos on the ground, Admiral Kaedar's armada, including the Mou'Llhei Star under Captain N'vae Celeneos, the Allecto under Captain Nerazachi and the Impavid under Captain Ramthor, ambushed the Dark Hand, and after a brutal broadside with the mighty Super Star Destroyer, sent it crashing into the surface of Achiliquin's moon. Once Maethoruin was secured in the Accuser's sickbay, Admiral Prentus ordered a retreat from the system—the Battle of Achiliquin had been lost.
While the battle of Cyranduas had been embarrassing from a symbolic standpoint, the Imperial defeat at Achiliquin sent ripples throughout the Empire. The Commission for the Protection of the Imperial Order (COPRIMOR) quickly set out quelling news of the disastrous defeat within Imperial space, blaming the defeat on the Republic's dishonourable tactics, and the falsified intervention of a popular Imperial scapegoat—the Unified Order of Cognalorilos. In Republic space, however, the reaction was quite the opposite, with some in the Republic media daring to call the surprise victories at Cyranduas and Achiliquin as a definitive turning point in the war. Such reports, while exaggerated, were somewhat reflected in renewed talk on Capricaerón for a new Republic campaign to capture, and hold, Imperial territory in the Core and Inner Rim. Achiliquin itself began the long process of rebuilding, with the Council, reeling from the destruction wrought by Maethoruin's forces, urging Senator Buroa to take a more aggressive stance toward the Empire in the reconstituted Republic Senate.