During the Imperial Era, the troop carrier known as the Sempre suffered an attack. Two squadrons of V-19 fighters, which had been stolen, and the frigate Castilus were the assailants. The Sempre was transporting Wookiee slaves at the time. The attackers inflicted damage upon the transport and then boarded it. The captain of the Sempre sent out a distress call. This call was picked up by the Imperial Star Destroyer Chimaera, the flagship of Commodore Thrawn. Thrawn suspected that Nightswan was the instigator of the attack. Imperial authorities instructed Major Ayer, the stormtrooper commander on the Chimaera, to keep the nature of the Sempre's cargo secret from the rest of the crew. However, Lieutenant Commander Eli Vanto and Commodore Thrawn both pressed to inspect the Sempre and found out what it was truly carrying.
Following the conclusion of the Clone Wars, the Galactic Empire took control of and enslaved the Wookiees residing on Kashyyyk, considering them to be little more than beasts. The xenophobic Galactic Empire sought to exploit the Wookiees due to their large, strong bodies, which made them suitable for performing labor in difficult and hazardous environments. The Empire completely dominated Kashyyyk, forcing the Wookiees to harvest wroshyr trees, grow food for the Imperial Military, and mine wroshite mineral deposits. Wookiee slaves were also sent to other locations to work in Imperial mines and on projects like the Death Star. In addition, Wookiees were also experimented on with weapons and medicines.
Prior to being transported off-world, Wookiee slaves were processed at space stations like Lansend Twenty-Six. They were then moved using modified troop carriers such as the Sempre. The rebel leader Nightswan and his organization became interested in freeing enslaved Wookiees as part of his insurgency against the Empire. Eight weeks before the raid on the Sempre, the insurgents stole the frigate Castilus and two squadrons of V-19 Torrent starfighters.
While the Sempre was traveling through space, the insurgents, using their frigate and V-19 Torrent starfighters, launched a surprise attack. The boarders forced their way onto the ship and freed the Wookiee slaves. The Wookiees, now free, joined the battle, slamming numerous stormtroopers and crew members against the walls. The Sempre's crew managed to send out a distress signal, which was intercepted by the Imperial Star Destroyer Chimaera, the flagship of the Chiss Commodore Thrawn. By the time the Chimaera reached the Sempre's location, one hour and 49 minutes later, the insurgents had already escaped with the Wookiees, and all the crew members were dead.
To keep the true nature of the Sempre's cargo a secret, Coruscant ordered Major Ayer, the stormtrooper commander, to board the Sempre while the Chimaera's crew dealt with the attacking ships. However, Commodore Thrawn and Lieutenant Commander Eli Vanto insisted on inspecting the transport ship. Vanto noticed that some of the fallen stormtroopers had blaster wounds, while others had injuries to their heads and torsos, suggesting they had been struck. Thrawn also observed that some of the bloodstains on the walls were higher than the victims. Vanto and Thrawn also discovered a blood mark left behind by the attackers.
Despite Major Ayer's objections, Thrawn insisted on examining the troop quarters. Thrawn and Vanto found that the racks in the troop carrier had been modified to accommodate Wookiees. Thrawn then concluded that the Sempre was transporting Wookiee slaves, information that the Empire wanted to remain confidential. Under Thrawn's command, the Chimaera then went to assist the besieged station of Lansend Twenty-Six, which was under attack by the insurgents seeking to liberate more Wookiee slaves. Thrawn's reinforcements were successful in defeating these insurgents.
The attack on the Sempre was first depicted in Timothy Zahn's 2017 novel titled Thrawn.