Once a respected Jedi Knight, Muur's desire for power and study of the dark side of the Force lead him, alongside Pall and XoXaan, to be exiled to the farthest reaches of the galaxy. The three arrived at Korriban, where they discovered the native Sith species and enslaved them to the dark side, becoming the first Sith Lords. Muur also possessed a powerful Sith talisman known as the Muur Talisman, which had the ability to turn anyone within close proximity of the wielder into mindless thralls under his command.
Muur intended to use an army of rakghouls to dominate the galaxy. However, Remulus Dreypa, a rival Sith Lord, created a specialized stasis pod to capture and neutralize Muur and the power of his Talisman.
Despite his defeat, Muur's spirit lingered on within the talisman, and had the ability to possess others. His ultimate goal became for someone to find his talisman, allowing him to possess the person and gain access to their Force-abilities. He got his opportunity a number of years later when Jedi Master Celeste Morne saved Padawan Zayne Carrick from the powers of the Talisman. Muur bonded with Morne and attempted to fully take over her, but Carrick trapped Morne within a stasis pod to prevent this.
Muur and Morne were trapped together in stasis for four thousand years, until Darth Vader came to the desert moon to find the Muur Talisman. Vader freed Morne, who, while not fully possessed by Muur, was still the host for his spirit. Muur taunted Morne to allow Vader to kill her so she could be free of him. At the same time, he whispered to Vader to take the Muur Talisman for himself and slay his master, Emperor Palpatine. Morne turned the stormtroopers that accompanied Vader into rakghouls, forcing him to escape, likewise forsake his desire to possess the talisman, as he believed that he would just become enslaved to Muur's will.
Eventually, around the time of Darth Krayt's Empire, Muur was able to escape from the desert moon by influencing Morne to capture the Star Destroyer Iron Sun, turning its entire crew into rakghouls, and flying into the Deep Core. This Star Destroyer was boarded by a Jedi strike force lead by Cade Skywalker, who took it to the planet Had Abbadon, which was the site of an Imperial base that Cade and his strike force wanted to attack to lure Krayt out.
Muur started to play mind games with the other Jedi, planting visions in their heads, causing some to want to possess the Muur Talisman, some to want to destroy it. Eventually, Muur used the confusion to imprison Cade and his entire team.
Eventually, Muur, speaking through Morne, contacted Darth Krayt on Coruscant, and proposed an alliance between the two of them, with Muur offering to give Krayt immortality. Krayt traveled to Had Abbadon along with his servants, Darth Stryfe, Darth Talon, and Darth Maladi. However, it was here that Muur revealed his true plan: to take over Krayt's body and turn all the other Jedi and Sith into rakghouls. After Krayt was defeated (but not killed) in combat against Cade and Morne, Cade, on Morne's urging, killed her. The talisman then crawled up Cade's arms, with Muur attempting to possess Cade. However, Cade overloaded the talisman with Force energy, destroying it and finally destroying Muur once and for all.