As part of the Galactic Empire's efforts to bring order during pacification operations inside a specific sector found within the Mid Rim Territories, around the time of 2 BBY, Imperial forces operating under the command of Moff Ahr initiated an assault. The target was insurgents who had established a base on Phelzepham, a moon located within that sector. Among the governor's military contingent was Darth Vader, the Sith Lord. Galactic Emperor Sheev Palpatine had instructed Vader to follow Ahr's directives precisely, with the intention of imparting a valuable lesson to the Moff. As Vader, leading a stormtrooper detachment, leveraged their superior numbers and approached victory, Ahr made the decision to test his authority over Vader by ordering the stormtroopers to retreat. Despite being unexpectedly left to fight alone, Vader successfully completed the mission, defeating the insurgents.
During the time known as the Imperial Era, the Sith Lord Darth Vader became well-known as the iron fist of his Sith Master, the Galactic Emperor Sheev Palpatine. However, when the Galactic Empire commenced an attack on the celestial body called Namzor, situated in the Mid Rim Territories at approximately 2 BBY, Vader went against a direct order given by Moff Ahr to halt his offensive. This resulted in the deaths of the insurgents he was targeting, along with the destruction of the coaxium reserves that the Empire sought to acquire. Because Ahr held the position of political leader responsible for pacifying the overall sector that he and Vader were assigned to, Palpatine concurred that his apprentice should not have eliminated the insurgent stronghold. Consequently, Palpatine commanded his apprentice to strictly adhere to every order issued by the governor, until he had fully grasped the intended lesson.
The politician wasted no time in testing his control over Vader by assigning him to missions that carried a high risk of the Sith Lord being killed. The first instance of this occurred when Ahr's Venator-class Star Destroyer arrived at another celestial body in his sector, Ahn Krantarium. Upon arrival, the governor deployed a group of stormtroopers to the surface, only to have the entire patrol annihilated by the formidable droids that had long since taken control of the terrain. After being informed that attacking the droids would be a suicidal undertaking, Ahr commanded that Vader be sent in alone. Ultimately, the Sith Lord prevailed over the droids, and in accordance with his master's instructions, he continued to display unwavering loyalty to the governor. Following this event, Ahr, Vader, and their forces proceeded to the moon of Phelzepham.
Phelzepham, another celestial object within Ahr's sector, had become a refuge for a different group of anti-Imperial insurgents. As part of the ongoing pacification efforts in the sector, Vader and a contingent of stormtroopers were sent from the governor's Venator onto the moon's surface to confront the group, who had established at least one anti-infantry turret within a rocky region. The Imperial Military experienced a successful attack; their forces had a numerical advantage over the insurgents, and Vader, utilizing the Force and wielding his lightsaber, spearheaded the stormtroopers' advance. As many of the insurgents sought protection behind the rocky terrain, Vader and his soldiers continued the fight. At least one insurgent and two stormtroopers were killed in action, and the Sith Lord used telekinesis to pushed back two other insurgents, as both sides exchanged blaster fire.
Meanwhile, aboard the Star Destroyer, Ahr was informed by an Imperial officer that the Empire would likely secure the moon within the hour. However, instead of maintaining their advantage in numbers, the politician decided to test Vader once more, ordering all stormtroopers to retreat, leaving the Sith Lord to deal with the insurgents on his own. Now standing alone, Vader persisted in the attack, despite being suddenly outnumbered and outgunned. In spite of this, Vader ultimately emerged victorious, although his armor was left aflame. Disregarding this, Vader returned to the bridge of Ahr's destroyer, where the governor acknowledged the Sith Lord's survival and victory as positive news, while simultaneously contemplating another test that would likely result in the demise of the Emperor's Fist.
Eventually, Ahr commanded his Star Destroyer to enter the Kankalo Belt Containment Zone, instructing Vader, who was departing the Venator in his personal starship, to eliminate the most significant threat he encountered within the asteroid field. Once again following the governor's orders to the letter, Vader turned back to the Venator after discovering a crustaceous creature. Reasoning that the governor had become the greatest threat to him, Vader returned to the bridge and observed as the creature breached the viewport, pulling the governor into the vacuum of realspace. Ahr and several other Imperials standing nearby were killed. Palpatine sensed the murder and judged that Vader, who stood tall on the bridge as stormtroopers and Imperial officers bowed to him, had learned the lesson he was meant to.
The attack on Phelzepham was depicted in the one-shot comic issue Age of Rebellion - Darth Vader 1 from Marvel Comics' Star Wars: Age of Rebellion series. The comic, written by Greg Pak and illustrated by Ramón Bachs, was released on June 26, 2019.