Westen became the captain of the Star Destroyer Relentless in 0 ABY after the previous captain, Parlan, failed in a mission ordered by Darth Vader. Parlan had been sent to capture Republican tactician Adar Tallon, who was hiding on Tatooine. However, Tallon escaped from Parlan's agent and joined the Rebel Alliance. Because of his mistake, Parlan was strangled by Darth Vader.
Westen knew what had happened to Parlan, and feared a similar end. This is why he worked hard to avoid failing people like Vader.
Westen had a chance to fight Adar Tallon, the man who escaped from his predecessor. Commanding the Relentless, Westen fought the new flagship of the Rebel commander, Silent Water. Tallon considered different tactics to use in the fight, and one of the factors he pondered was that Westen was not at all like Parlan.
Later, Moff Ravik, who had been captured by Rebels and being transported to safe word on board of ship Celestial, managed to escape from his cell. Working alongside commander Bane Nothos and other prisoners, Ravik had staged a revolt, taking the control of the ship from her previous captain, Alain Gryphon. Ravik sent a distress signal to the nearest Imperial base. The Relentless was sent to pick up Ravik, who had also the coordinates of the Rebel safe world.
However, when Westen arrived, the Celestial entered hyperspace because of a final effort of captain Gryphon. Westen, afraid of Imperial reprisals, could not afford to lose Ravik. He sent a whole wing of TIE/LN starfighters to the area, to track any possible hyperspace lane that the Celestial might have used.
While the fighters were scouting, a Rebel reinforcement ship arrived; it was the Long Shot. The Shot collided with one TIE and then began a conflict with Westen's forces. Understanding that she was at a disadvantage, the Shot began a hyperspace jump, and Westen wanted to use it to find the lane.
However, the hyperdrive on the Shot had been damage because of the initial collision with the TIE, and the Celestial also had had a damaged hyperdrive. Due to this, their hyperlanes were completely different and alien, and Westen could not track them.
Westen returned to his base, empty-handed. His mission had been a failure. Vader then executed Westen and replaced him with Dorja in the bridge of the Relentless.