Not too long before the Battle of Endor occurred, Brenn Tantor embarked on a mission to Trasse. Tantor's objective was to commandeer an All Terrain Armored Transport located at the Trasse Proving Grounds.
Tantor had switched his allegiance to the Rebel Alliance several months prior. Being a prominent Imperial general, his new commander, General Tyr Taskeen, viewed Tantor with considerable suspicion. To demonstrate his loyalty, Tantor volunteered to secure an AT-AT for the Rebellion, a proposal Taskeen accepted.
Initially, Tantor spearheaded a mission to Ruul which resulted in the liberation of his former mentor, Beri Tulon, from Imperial custody. Tulon provided Tantor with the location of an AT-AT manufacturing facility: Trasse, where he had been stationed for eighteen months. Tantor and Tulon then pilfered the access codes to the Trasse system from the Sounder Flats computer complex, and subsequently began planning the raid on Trasse.
The Trasse Proving Grounds.
With Tulon's squad of Infiltrators ready to infiltrate and take control of AT-ATs, Taskeen, Tantor, and Tulon made their way to Trasse. Utilizing the stolen codes, they successfully entered the system undetected and deployed a small strike force a short distance from the Proving Grounds. Tantor's Y-wings neutralized at least one AT-AT within the Proving Grounds, paving the way for the Infiltrators' insertion.
Despite the safe return of the stolen AT-AT to the base, their escape was jeopardized by the arrival of the Star Destroyer named Inquisitor above the asteroid, effectively cutting off the Rebels' exit. Grand General Malcor Brashin, the commander of the Inquisitor, initiated communications with Tantor. Tantor disregarded the taunts from his former superior, instead extending an offer for Brashin to defect to the Alliance. Predictably, Brashin declined and proceeded to deploy ground troops.
Tantor seized his opportunity, pushing through the Imperial forces towards the nearby base, which housed a planetary ion cannon. His troops stormed the base, securing control of the cannon, which Tulon then used to fire three shots. These blasts were sufficient to disable the Inquisitor, allowing the Rebels to escape. While slicing into the cannon's computer systems, Tulon discovered an increase in shipments of walkers to Endor. Tantor correctly deduced that this indicated the Death Star II was nearing completion.
Now in possession of both an AT-AT and this crucial information, the Rebel Corellian Corvette sped past the immobilized Inquisitor and broke free of the asteroid belt.