Cassus Fett, a Mandalorian Field Marshal from the era of the Mandalorian Wars, held a prominent position. Serving as an aide-de-camp to Mand'alor the Ultimate, the leader of the Mandalorians, Fett wielded considerable influence throughout their society.
Fett was a leading figure in the rise of the Neo-Crusader movement. This faction within Mandalorian society embraced a more structured organization and standardized armor, a departure from the previously dominant Mandalorian Crusaders. By consolidating control over the clans and establishing a system for integrating conquered populations into the Neo-Crusaders, Fett effectively transformed the Mandalorians into a formidable war machine with the ambition of galactic conquest.
Operating in the Outer Rim, Fett engaged the forces of the Galactic Republic in a series of conflicts. This included a massacre on Cathar that decimated the Cathar species. Before initiating a full-scale invasion of Republic territory along three distinct axes in 3963 BBY, he orchestrated a number of smaller skirmishes at the edge of Republic space to assess their defenses.
Fett participated in the initial Battle of Vanquo, spearheaded the siege of Taris and its eventual capture, devised a plan to invade the planet of Alderaan, and eliminated the captain of a Republic flagship during the Battle of Jaga's Cluster. After becoming the galaxy's most wanted individual, Fett was presumed deceased by the time the Jedi Civil War erupted. Years later, two other members of his Clan, Jango and Boba Fett, would honor their clan's legacy with their impressive capabilities.
The origins of Cassus Fett remained largely shrouded in mystery across the galaxy. Republic Intelligence lacked information about his background, and even his fellow Mandalorians possessed limited knowledge, although it was known he was Human. Following the conclusion of the Great Sith War and the subsequent defeat of the Mandalorians, a new leader, known as Mandalore the Ultimate, ascended to power. Mandalore, seeking to redefine Mandalorian identity, assembled a group of skilled advisors, including Fett, who possessed talents beyond mere combat. Fett rose to become Mandalore's aide-de-camp and chief strategist, wielding significant influence both on and off the battlefield. He played a crucial role in transforming the disorganized Mandalorian forces into a well-structured and efficient organization.

Unlike previous Mandalorians who simply conquered worlds without integrating them into Mandalorian culture, Fett recognized the strategic value of incorporating conquered populations. He and Mandalore the Ultimate observed the emergence of a cult among the Mandalorians, known as the Neo-Crusaders. This group, based on the planet Mandalore, diverged from Mandalorian tradition by adopting uniform armor and an unconventional command structure and division of labor. Fett persuaded Mandalore to embrace their ideals, and together they promoted the Neo-Crusader movement. While Mandalore mandated the widespread adoption of the Neo-Crusader system, it was Fett who oversaw its implementation. He adapted the Neo-Crusader cult's organizational system for large-scale application and integrated Neo-Crusaders into the clans to disseminate their ideology. Fett believed that standardized uniforms would facilitate control over the increasingly unruly Mandalorians. He also advocated for color-coded armor to denote rank, enabling new recruits to easily identify authority figures. While the lowest ranks wore blue armor, Mandalore's top advisors donned gold-colored suits, and Fett was among the first to adopt this armor as a symbol of the collective identity all Mandalorians should embrace. Before the invasion of the Galactic Republic commenced, Fett's system for assimilating conquered cultures into the Mandalorian ranks was fully operational.
However, Fett faced opposition from within the Mandalorian ranks, particularly from individuals such as Demagol and Pulsipher, whom he considered rivals. Demagol, a Mandalorian scientist, was another advisor in Mandalore's inner circle, and Fett viewed him as a competitor for Mandalore's attention. Demagol, in turn, irritated Fett by refusing to wear Neo-Crusader armor.
In the years leading up to the Mandalorian Wars, but before the invasion of the Republic, the Mandalorians were a significant presence in the Outer Rim. During this period, Fett conquered several unaligned worlds, establishing his reputation as a skilled military commander. In 3976 BBY, Fett led a devastating attack on the planet Cathar, seeking retribution against the native Cathar for their past resistance to the Mandalorians. Leading an army of warriors on Basilisk war droids, Fett drove the Cathar to the brink of extinction. The surviving Cathar were rounded up and forced into the planet's sea, where Fett prepared to execute them. A lone Mandalorian stood with the Cathar, pleading with Fett to spare them. Fett remained resolute, and his ships unleashed their firepower upon the Cathar and the lone warrior. The massacre was so horrific that the remaining Cathar were afraid to speak of it for years. The Republic concealed the event, fearing that its disclosure would destabilize reconstruction efforts following the Sith War. The disappearance of the Cathar remained a mystery for over a decade.

Shortly before the full-scale invasion of the Republic, Fett played a key role in orchestrating the False War, a brief conflict in the Outer Rim with Republic forces. This involved feigning weakness to gauge the strength of the Republic Navy, which had been revitalized after the Sith War. Fett also developed the next phase of the Mandalorian military strategy: a three-pronged invasion of the Republic known as the Onslaught. Launched in 3964 BBY, this assault capitalized on the recent turmoil within the Republic, including the murder of four Jedi Padawans on Taris and the escape of Zayne Carrick, the Jedi falsely accused of the crime. The planet Vanquo was targeted to divert Republic forces, paving the way for an assault on the strategically important world of Taris. Fett and Mandalore monitored the invasion from Mandalore's flagship, observing the progress of the Vanquo assault while preparing to advance on Taris. Fett established an orbital screen around Vanquo, enabling the main force to engage the major settlements on the planet's day side, while shock troopers landed on the night side to eliminate the mining camps. Before the Republic could mount a defense, Fett and the Mandalorians had seized control of several worlds.
In addition to reorganizing the Mandalorian forces, Fett also revolutionized Mandalorian military production. He established War Forges near the Neo-Crusader staging grounds on Jebble, Tarnith, and other worlds. These forges quickly became operational, providing the Mandalorian advance with the necessary armor and weaponry for new recruits.
By 3963 BBY, the attack on Taris was in full swing. Following the collapse of the Republic's defensive Jebble-Vanquo-Tarnith line to the Mandalorians, Admiral Jimas Veltraa regrouped his forces around Taris. However, they were ultimately defeated, and the Mandalorians laid siege to the planet. As Mandalorian vessels entered orbit around Taris, continuously reinforcing their numbers and adding to the planetary bombardment, repeated attempts by Tarisian planetary defense fighters to break the siege proved futile. The Mandalorians were also engaged on other fronts, laying waste to the world of Serroco in an attack orchestrated by Fett. Following reports of Demagol's death at Flashpoint Stellar Research Station—although he had actually been captured by the Republic—Fett's influence grew. Mandalore personally dispatched Fett to Taris to assist with the siege and oversee the logistics of such a large-scale planetary conquest. This experience would prove valuable in their eventual invasion of Coruscant, which Fett envisioned as part of their campaign.
Eventually, the time came to seize the planet. The Mandalorians initiated their ground assault by landing troops in Taris' polar regions, followed by landings in the equatorial regions, ultimately leading to a planet-wide invasion. With what a Tarisian reporter described as "macabre efficiency," Fett led his troops, on foot or on Basilisk war droids, to conquer the Upper City. Swoop gangs briefly engaged the Mandalorians before retreating into hiding, and soon even news broadcasts were interrupted as the conquest swept across the planet. With the Mandalorians' victory on Taris secured, Fett made an announcement across the world on the Taris Holofeed. Informing the people of Taris that the Republic had failed them, Fett released them from their allegiance to the Republic and the Supreme Chancellor. As he had done on other conquered worlds, Fett offered the citizens of Taris a choice between joining the Neo-Crusaders or working in their factories as de facto slaves. With a final warning of death to those who would betray the Mandalorians, Fett deactivated the Holofeed.

As the siege of Taris progressed, Fett observed the growing congestion caused by the invasion force's vessels, appropriated vehicles, and ships attempting to escape. His own forces were often obstructed, resulting in the loss of valuable resources. This situation threatened to delay the Mandalorians for months, but from his headquarters in the former Jedi Tower, Fett effectively managed traffic control on and around the planet. Simultaneously, he continued to wage war against the Taris Resistance. As the Lower City of Taris was still being conquered, Fett ordered the city's skylanes to be kept clear for landing ships, and he personally directed those above the planet to their designated destinations.
When news of Fett's headquarters in the Tower reached the Resistance, they devised a plan to assassinate him. Their strategy involved detonating demolition charges in the building's foundations, but to ensure Fett's presence during the attack, they planned to infiltrate a reconnaissance team into the Tower first. Meanwhile, Fett studied the Resistance strongholds in the Lower City and identified their hiding places. He withdrew his men from the Tower and launched an attack on the Resistance base, preventing the infiltration team from completing their mission. They destroyed the Tower regardless, but the remaining Resistance forces were forced to retreat into the Taris Undercity.
Shortly after the invasion of Taris, the Republic began broadcasting propaganda on Mandalorian frequencies in an attempt to undermine morale and dissuade Neo-Crusader converts. Fett contacted Mandalorian officer Koblus Sornell to discuss the transmissions and any potential information they contained. As the Mandalorians prepared an invasion of the planet Zongorlu from Taris, the Republic's broadcasts attempted to discourage them with reports of deadly conditions and dangerous inhabitants on the world, as well as the existence of massive Republic ships. Fett instructed Sornell to create a mobile signal station for the invasion fleet to communicate with him upon arrival. Sornell informed Fett of the problems the transmissions were causing within the newly recruited ranks, with distracted warriors experiencing accidents, some of them fatal. Upon invading Zongorlu, Sornell and the fleet discovered that the broadcasts had been a hoax; the world was temperate, its native species was a sessile plant, the massive ships were nonexistent, and the broadcasts originated from a small transmission station on the planet. Sornell reported her findings to Fett, who ordered her to hold station for a week, a significantly shorter duration than the intended operation. Although the transmissions had ceased upon the invasion, they soon resumed, this time offering the Mandalorians a deal: in exchange for peace, the Republic would grant them a portion of taxation from the worlds they had invaded. Sornell rejected the offer, and Fett subsequently contacted her, reprimanding her for not waiting. Sornell defended her decision.
Following the battle with the Resistance, Fett commandeered several troop transports and traveled to Jebble, a Mandalorian-controlled ice world used as a staging area where captured criminals were being trained as Neo-Crusaders. With these new recruits, he planned to launch an invasion of the Core World Alderaan, deep within Republic territory. However, before Fett could arrive, Pulsipher—another Mandalorian rival—reached Jebble with a prize he had discovered in the Undercity of Taris: the Muur Talisman, a powerful Force artifact. Upon donning the Talisman, Pulsipher inadvertently unleashed a plague that transformed its victims into Rakghouls. Soon, the Mandalorians stationed on Jebble were succumbing to the transformation.

Meanwhile, the Jedi fugitive Zayne Carrick had stowed away on Pulsipher's transport and was now on Jebble. After evading the waves of Mandalorians and Rakghouls, Carrick located a communications station and contacted the approaching Mandalorian fleet. He warned Fett about the plague outbreak and advised him not to land, but Fett, aware of Carrick's involvement in the assassination attempt, was skeptical. Pulsipher, wielding the Talisman, had gained control over the Rakghouls and intended to usurp Fett's position by attacking the landing transports and spreading his influence throughout the galaxy. However, Pulsipher was killed, and Carrick escaped the planet with the help of his friends. Fett arrived as Carrick was departing and, convinced of the Jedi's sincerity, bombarded the planet's surface with missiles. He then contacted Carrick and acknowledged his debt to the Jedi.
Later that year, the truth about the Cathar massacre came to light. A charismatic Jedi known as The Revanchist led the Jedi to Cathar in search of answers. Upon discovering the mask of the lone Mandalorian who had stood against Cassus Fett, the gathered Jedi experienced a vision of Fett's massacre. Driven by the fate of the Cathar, the Jedi, led by Revan and his friend Malak, promptly joined the war against the Mandalorians.
Shortly thereafter, Zayne Carrick, searching for the slaver Dace Golliard—and through him, Demagol, who was still alive—had fellow Mandalorian Rohlan Dyre, also presumed dead, contact Fett. Carrick, banking on Fett's willingness to repay his debt and his animosity towards Demagol, devised a false Mandalorian strike against an asteroid mining facility near the Ithor system. While Carrick anonymously alerted the Republic's Defense Ministry to the "attack," Fett and the Mandalorians sent an unscrambled message for Golliard to intercept. Fett and his fleet arrived in the asteroid field as planned, confronting Admiral Saul Karath of the Republic while Golliard, anticipating a battle from which he could scavenge, waited in hiding nearby. As Fett and the Mandalorians approached Karath's fleets and Karath prepared to engage, the Mandalorians suddenly jumped to hyperspace, leaving Golliard at the mercy of the Republic and Carrick. Fett contacted Carrick after the engagement to inform him that the Mandalorians' debt had been repaid. He instructed Dyre to keep his survival a secret and expressed his hope that the group would be able to eliminate Demagol.
Fett later engaged Revan and Malak in the Battle of Jaga's Cluster, where he gained notoriety for killing the captain of a Republic flagship. Fett eventually became the most wanted individual in known space. The Mandalorian Wars ultimately ended in a Republic victory, and by 3956 BBY, Cassus Fett was presumed dead.
Fett was a member of the Mandalorians' Clan Fett and had at least one son. A suit of Fett's armor eventually ended up in the possession of the merchant Adum Larp of Dantooine, and his heavy blaster pistol came into the possession of B'ree, a Czerka Corporation merchant on Korriban.
During the Mandalorian Civil War between the Mandalorian Tra'tade and the Hidden Chain, Durn Wynnward reflected on Fett's influence regarding the Neo-Crusaders and their rise under his leadership.

Cassus Fett possessed unparalleled tactical acumen and a reputation as a master of command. A methodical individual, he masterminded many of Mandalore's victories during the war and held the rank of Field Marshal by the time of the invasion of Taris. He was fluent in several languages, including High Galactic and Mando'a. Fett's influence permeated all aspects of Mandalorian affairs, reshaping their entire culture. His contributions to Mandalorian strategy extended to mundane matters such as traffic control. Fett's concepts of exerting control over the Crusaders and prioritizing logistics were foreign to the Mandalorians, but they significantly aided the Mandalorian cause, transforming them into a force capable of challenging the Republic's dominance in the galaxy. Fett also employed trickery as a tactic, which fellow Mandalorian Rohlan Dyre considered dishonorable and a reflection on Mandalore the Ultimate for relying on it. While Fett recognized that his system was unconventional for the Mandalorians and their nomadic lifestyle, he believed it would lead them to ultimate victory. Fett was known among his fellow Mandalorians for his ruthless tactics, willing to sacrifice millions to advance the greater cause of his people. He readily ordered the mass killings of populations for revenge, to eliminate potential threats, or to uphold Mandalorian honor. Republic propaganda portrayed Fett as Mandalore the Ultimate's enforcer.
Fett viewed the Mandalorians as a new order, replacing the old order of Republic rule. He presented himself as a liberator of worlds, arguing that the Republic's inability to defend them rendered it unworthy of their allegiance. He offered the path of the Neo-Crusaders, promising them a share in the Mandalorians' victory. While he offered peace to those who joined the Mandalorians, he would not tolerate betrayal and promised death to those who feigned loyalty. Fett was wary of his enemies, but once convinced of their sincerity, he was willing to incur debts and repay them accordingly.
Fett had few peers or friends. He harbored rivalries with other Mandalorians, such as Demagol and Pulsipher, and was pleased to learn of their reported deaths. Even Mandalore's appreciation of Demagol did not deter Fett from contributing to his demise. For his part, Pulsipher held Fett in low regard and resented the amount of resources Mandalore entrusted to him.
To demonstrate his support for the color-coded armor system he helped implement, Fett wore a suit of yellow Neo-Crusader armor, initially reserved for Mandalore the Ultimate's closest advisors. Fett also possessed at least one other suit of armor, colored purple. He wielded a versatile heavy blaster pistol, which he was rarely seen without.
Fett could authorize Mandalorian Kandosii-type dreadnaughts to deploy fission weapons, which were used extensively during the war. He sometimes led soldiers into battle mounted on Basilisk war droids, as he did on Cathar.

Cassus Fett's initial mention occurred within Star Wars: Knights of the Old Republic. In this game, players have the opportunity to acquire his armor and blaster from two distinct merchants; these items feature flavor text that offers concise details about Fett. Intriguingly, the game's files contain two separate items named "Cassus Fett's Battle Armor," but only one is actually implemented in the game, while the unused version boasts a unique texture.
Although Fett's visual debut happened in the Knights of the Old Republic 8 comic, this fact remained obscure for a period because the comic itself did not provide explicit identification. When John Jackson Miller, the creator behind the Knights of the Old Republic comic series, faced the question of whether the Mandalorian character was indeed Fett, he responded by suggesting that the "text speak for itself," choosing not to disclose the information through other channels. Fett's name was first explicitly used within the series in The Taris Holofeed: Invasion Edition section of issue 18, and a visual association was first made in the subsequent The Taris Holofeed Special Proclamation section of issue 21, which repurposed a panel from issue 8 to depict Fett. Miller ultimately confirmed that the Mandalorian in issue 8 was indeed Fett in the notes for the Star Wars: Knights of the Old Republic Handbook on his personal website.
Furthermore, Fett was given a profile in the Handbook. The images taken from issue 8 were deemed too small for inclusion, and his appearance in issue 23 had not yet been released, leading to the use of a red-armored Mandalorian from issue 20, recolored in gold, to represent him. However, the character himself does not make an appearance in issue 20.
Miller has drawn comparisons between Fett and Darth Vader, believing that both characters are most effective when used sparingly. Many instances of Fett's presence in the series are limited to name references, focusing on how other characters react to his name.
Miller dedicated a significant amount of effort to pinpointing the appropriate tone for Fett's speech in The Taris Holofeed Special Proclamation, where the character announces the Mandalorian takeover of Taris. During a trip to New Mexico, he found inspiration in a speech delivered by General Stephen W. Kearny, who proclaimed New Mexico as part of the United States. Although the context differed from Miller's writing, he utilized it as a general guide for the concise nature of Fett's announcement.