Attack on the Sempre During the Imperial Era , the troop carrier " Sempre " was attacked by two squadrons of stolen V - 19 fighters and the frigate Castilus while transport ing Wookiee slaves . The attackers damaged and boarded the transport . " The Semipre " captain transmitted a distress call that was received by the Imperial Star Destroyer " Chimaera " , Commodore Thrawn ' s flagship . Thrawn suspected Nightswan to be behind the attack . Imperial authorities ordered the " Chimaera " stormtrooper commander Major Ayer not to tell the " Chimaera " crew that the " Sempre " was transporting slaves . However , Commodore Thrawn and Lieutenant Commander Eli Vanto insisted on inspecting the " Sempre " and discovered its true cargo . Prelude . Following the Clone Wars , the Galactic Empire subjugated and enslaved the Wookiee s of Kashyyyk , regarding them as beasts . The xenophobic Galactic Empire wanted to exploit the Wookiees because their massive , robust physiology allowed them to work in harsh and dangerous conditions . Kashyyyk was completely subjugated by the Empire with the Wookiees being forced to cut wroshyr tree s , produce crops for the Imperial Military , and extract wroshite mineral deposits . Wookiee slaves were also exported offworld as slave laborers in Imperial mines and projects such as the Death Star . In addition , Wookiees were also used as test subjects for weapons and medicines . Before being shipped offworld , Wookiee slaves were taken for processing at space station s such as Lansend Twenty - Six . They were then transported in modified troop carrier s such as the " Sempre " . The insurgent leader Nightswan and his operatives took an interest in liberating enslaved Wookiees as part of his insurgency against the Empire . The insurgents stole the frigate " Castilus " and two squadrons of V - 19 Torrent starfighter s eight weeks prior to raiding the " Sempre " . The raid . The insurgents ' frigate and V - 19 Torrent starfighters ambushed the " Sempre " while it was traveling trough space . The boarders stormed the ship and liberated the Wookiee slaves . The liberated Wookiees joined in the fight and slammed several stormtrooper s and crew members against the bulkheads . The crew of the " Sempre " picked up a distress signal that was intercepted by the Imperial Star Destroyer " Chimaera " , the Chiss Commodore Thrawn ' s flagship . By the time the " Chimaera " arrived at the " Sempre " coordinates one hour and 49 minutes later , the insurgents had escaped with the Wookiees and all the crew had been killed . Aftermath . To maintain the secrecy of the " Sempre " true cargo , Coruscant directed the stormtrooper commander Major Ayer to board the " Sempre " while the " Chimaera " crew dealt with the attacking ships . However , Commodore Thrawn and Lieutenant Commander Eli Vanto insisted on inspecting the troop ship . Vanto observed that several of the fallen stormtroopers had blaster burns while others had injuries to their heads and torsos consistent with being hit . Thrawn also observed that some of blood marks on the bulkhead were higher than the victims . Vanto and Thrawn also found that the assailants had left a blood mark behind . Despite Major Ayer ' s protests , Thrawn insisted on inspecting the troop quarters . Thrawn and Vanto discovered that the troop carrier racks had been modified to accommodate Wookiees . Thrawn then deduced that the " Sempre " was transporting Wookiee slaves ; information that the Empire wanted to keep secret . Under Thrawn ' s command , the " Chimaera " then went to aid the beleaguered station of Lansend Twenty - Six , which was being attacked by the insurgents who wanted to free more Wookiee slaves . Thrawn ' s reinforcements succeeded in defeating these insurgents . Behind the scenes . The Attack on the Sempre first appeared in Timothy Zahn ' s 2017 novel " Thrawn " , a Canon ical origins story of the titular character .