Cassus Fett was a Mandalorian Field Marshal who lived during the time of the Mandalorian Wars. Fett was an aide-de-camp to Mandalore the Ultimate, leader of the Mandalorians, and he was highly influential in areas throughout Mandalorian society. Fett spearheaded the promotion of the Neo-Crusader movement, a sect of Mandalorian society with a more regimented structure and standardized armor than the hitherto dominant Mandalorian Crusaders. By exerting control over the clans and providing a means of converting conquered cultures into Neo-Crusaders, Fett turned the Mandalorians into a war machine intent on galactic conquest.

On the Outer Rim, Fett engaged the forces of the Galactic Republic in a series of battles, including a massacre of the Cathar people. He engineered a number of minor engagements at the edge of Republic space to determine the strength of the Republic, before launching into a full scale invasion of Republic space along three fronts in 3963 BBY. Fett was present at the preliminary Battle of Vanquo, led the siege of Taris and eventual conquest of that world, planned an invasion of the planet Alderaan, and killed the captain of a Republic flagship at the Battle of Jaga's Cluster. After becoming the most wanted man in the galaxy, Fett was presumed dead by the time of the Jedi Civil War. Many years later, his descendants, Jango and Boba Fett, would live up to his legacy.