The Ghost Prison incident was a military coup that occurred in the early days of the Galactic Empire's reign. The coup was orchestrated and instigated by Gentis, the Headmaster of the Imperial Academy and a hero of the recently-ended Clone Wars. After serving Supreme Chancellor-turned-Galactic Emperor Palpatine throughout the Clone Wars and the early Imperial Period, General Gentis grew weary of war and angry over the continued deaths of thousands of young Imperial soldiers. In 19 BBY, he resolved to betray the Imperial establishment by enlisting the first class of Imperial Academy graduates in a conspiracy to overthrow the Emperor and seize control of the Empire, thereby putting an end to destructive campaigns of oppression and expansion.
The coup began on the Imperial capital world of Coruscant, the evening of the Academy's graduation ceremony. Gentis-allied saboteurs and bombers detonated explosives at critical targets throughout Galactic City, while the Empire's newest class of officers disguised themselves as terrorists and attacked stormtroopers outside of the Imperial Palace, General Gentis and several of his most trusted students personally met with Emperor Palpatine in his throne room, where they detonated bombs containing the deadly Aorth-6 virus. The bioweapon quickly killed all of Palpatine's defenders, but the Emperor was spared from death by his own Force abilities and the intervention of Imperial Moff Trachta. Although Supreme Commander Darth Vader and the faithful Lieutenant Laurita Tohm defeated the attackers outside of the Palace, Emperor Palpatine's weakened condition and Headmaster Gentis's influence on Coruscant forced the loyalists to flee the planet.
Lord Vader and Moff Trachta agreed to take Emperor Palpatine to a location far removed from General Gentis's influence. Calling upon his past experience as a Jedi, the Dark Lord suggested The Prism, a secret prison facility previously maintained by the Jedi Order and entirely unknown to the rest of the galaxy. Vader, Tohm, and Trachta ferried Palpatine to The Prism, where he was rehabilitated; meanwhile, Gentis consolidated power on Coruscant and moved forward with his plan to seize total control. Given that he was uncertain of the Emperor's survival, he maintained the facade of a terrorist attack and instead decided to target Grand Moff Wilhuff Tarkin—one of the Empire's most prominent military and political leaders and the head of the secretive Death Star project.
On The Prism, Vader and Tohm hatched a plan to retake Coruscant by recruiting an army comprised of the prisoners in the facility. After instigating violent riots to cull the weak and find the strongest of the inmates, the Imperials selected thirty-three former criminals to pardon and reform into a pro-Palpatine fighting force. Their plan reached fruition when the loyalists were able to intercept Governor Tarkin's shuttle en route to Coruscant and confront Gentis. A fierce battle erupted between Palpatine's forces and the insurgent officers upon the shuttle's arrival on the planet, but in spite of the revolutionaries' superior numbers, they were quickly overcome. Emperor Palpatine personally killed Headmaster Gentis during the battle, finally ending the coup and reclaiming Coruscant. In the aftermath of the conflict, later dubbed the "Ghost Prison incident," the surviving conspirators were rounded up and executed by firing squad and the events of Gentis's coup were deemed classified by the Imperial government. Trachta and Tohm were promoted to the positions of Grand Moff and Admiral, respectively, and the remaining prisoners from The Prism were killed.
In 19 BBY, a few weeks after the rise of the Galactic Empire, General Gentis attended a military meeting on Imperial Center. While seeking to escape the droning of the various Moffs, he discovered the Imperial Crematorium, and watched hundreds of dead Imperials be incinerated every minute. Realizing that even though the war had ended, the Emperor Palpatine's warmongering ways would never cease, he quickly set about seeking a way to end the senseless killing. The fact that the Emperor's warmongering ways also cost him all of his sons except for Caul only solidified his desire to end it.
A few months later, the cadets of the graduating class of the Raithal Academy arrived on Coruscant. They were part of six different academies from around the galaxy who came to the capital in order to be part of the first graduating class of Imperial officers. Their training—like that of the Empire's clones and other military personnel—had been accelerated from the usual years to only a few months. Gentis, a hero of the Clone Wars, presided over the ceremonies. Laurita Tohm was the valedictorian of the Raithal Academy, and thus was given the rank of lieutenant by Gentis.
Lieutenant Tohm was soon selected to be a part of Grand Moff Wilhuff Tarkin's fleet, but Headmaster Gentis had other plans for the young officer. He ordered Ensigns Caul and Shens to keep Tohm "out of harm's way." Tohm's friends locked him inside a prototype Imperial starfighter, and for a time, he assumed it was a practical joke. In reality, this was the first small step in Gentis's coup, as he believed that the promising officer would draw too much attention due to his prominence and deformities. The Headmaster, in truth, cared deeply about those he trained, and orchestrated the coup in part because he knew their new Emperor would never cease throwing Imperials to their deaths.
Explosions rocked the cityscape for kilometers around, eliminating seventy-two Imperial targets within minutes. The officer's club, various stormtrooper garrisons, the Comm Towers, and a supply depot were all destroyed. Imperial Plaza became ground zero in the battle that erupted, as "terrorist" forces swarmed through the city streets, fighting with the Empire's clone forces. The stormtroopers were losing the battle until the Supreme Commander, Darth Vader, appeared, rallying the troops and driving the enemy forces back. In the madness, Tohm had escaped from the prototype starfighter by engaging its ejection seat and landing in the midst of the battle. Lieutenant Tohm joined the stormtroopers in following Lord Vader, but when one of the terrorists grabbed his ankle, he bent and removed the man's mask, revealing his friend Shens. Shens begged Tohm to help them kill Palpatine, the main target of their assassination attempts that night.
Palpatine escaped to a hidden Jedi prison, the Prism.
As Shens lay dying, he then asked Tohm to kill Vader, but Tohm told his friend that he should have included him earlier, and shot Shens in the head. The young lieutenant then bowed to the Dark Lord of the Sith, who instantly respected his bravery and loyalty. As the pair made their way from the plaza, another explosion tore through the nearby buildings, unleashing a powerful necrotic virus known as Aorth-6. Dozens of Imperials, including Royal Guardsmen, were killed by the aerosolized virus, which liquefied them from the inside out. Lieutenant Tohm donned a breathing device that he had taken from the TIE fighter when he ejected and followed Lord Vader inside. There they found a mortally infected Emperor Palpatine and Moff Trachta. Palpatine was using the dark side of the Force to keep the virus at bay, but was losing the battle even with his formidable powers. Vader accused Trachta of being in league with the conspiracy, but the Moff vehemently denied it, saying they needed to get the Emperor into hiding; fear of his power was the only thing keeping the Empire together. The trio escorted the weakened Sith Lord to a secret hangar below the Imperial Palace, where they loaded him aboard a stealth transport.
The team stopping the riot.
Using information gleaned from the Jedi Temple's High Council Chamber, Vader deduced that the only safe haven for the sick Palpatine was at the Prism, a prison facility located near Diab's sixth moon at the Diab system that was kept secret by the Jedi Council from even the Supreme Chancellor.
Lacking loyal or trustworthy soldiers, Tohm suggested to Vader that the prisoners of the Prism be released and allowed to fight for them, which would allow them to retake Coruscant from Gentis. The Dark Lord amended the young officer's idea, and rather than simply releasing the prisoners, he forced 207 of them to fight one another, as well as the lieutenant and himself, for the right to serve the Galactic Empire. The battle was vicious and wild, with casualties mounting quickly. Separatist sniper Shonn Volta allied herself with Tohm after saving his life from General Ur'Loach, while Vader defeated Nax Cirvan, a fallen Jedi, who then submitted to the Sith Lord.
In the end, only thirty-three prisoners survived in fighting condition, including Volta and Cirvan. Vader ordered Trachta to tell the survivors what it meant to serve the Empire. Trachta then told the survivors that their crimes were now forgotten, but their reasons for committing them were not—they would now serve an institution not beholden to the "selfish" senators of the Republic, nor the Jedi Order. The prisoners knelt before the Moff and his allies, becoming Imperials through their submission.