Horak-mul existed as a Sith Lord in the time period preceding the Great Hyperspace War. Being a member of the Sith species, Horak-mul originated from a region of space referred to as the Sith Empire. Following the demise of the Dark Lord of the Sith Marka Ragnos, Horak-mul chose to support Ludo Kressh, who was competing for Ragnos's title. Kressh's primary adversary was Naga Sadow, an ambitious and progressive Sith Lord. In the year 5000 BBY, Sadow orchestrated an assault on the Sith Empire, framing the Galactic Republic for the act. He then used this event as justification to proclaim himself the new Dark Lord of the Sith. This action resulted in a division between Sadow and Kressh, with Horak-mul aligning himself with Kressh. Subsequently, Sadow staged another attack, deliberately leaving behind evidence that implicated him in the plot. Upon discovering this evidence, an enraged Kressh instructed Horak-mul, along with another ally named Dor Gal-ram, to mobilize their forces and engage Sadow. The resulting conflict was a skirmish that occurred at Khar Delba, where Horak-mul and his comrades were outmaneuvered and defeated by Sadow. During the battle, Sadow ordered the crew of Horak-mul's flagship to betray and assassinate him, leading to the deaths of both Horak-mul and Gal-ram.
In the era before the Great Hyperspace War, Horak-mul served as a Sith Lord, originating from a region of the galaxy known as Sith Space. Horak-mul was involved in the Subjugation of the False Apprentice during his lifetime, and he collaborated with fellow Sith Dor Gal-ram to create a series of tablets. In 5000 BBY, Marka Ragnos, the long-reigning Dark Lord of the Sith, died after a century of imposing rule. Naga Sadow and Ludo Kressh, two Sith Lords, emerged as contenders to fill the resulting power gap. Horak-mul chose to support Kressh, the more traditional Sith Lord, over Sadow, an ambitious Lord who sought to expand the Sith Empire's territory. During the Funeral of Marka Ragnos, the Starbreaker 12, a ship from the Galactic Republic, landed in the Valley of the Dark Lords on Korriban, where the funeral was being held. The ship's crew, Gav and Jori Daragon, two Humans, were apprehended. A significant debate arose concerning the fate of the Humans. Kressh advocated for their immediate execution, while Sadow argued for keeping them alive, believing they possessed valuable information.
Eventually, Sadow orchestrated a Republic attack on the Sith Empire, inciting the Sith into a state of frenzy. Following this attack, during a meeting of the Sith Council on the planet Ziost, Sadow declared himself the new Dark Lord of the Sith. Horak-mul silently observed as the majority of the other Sith Lords expressed their support for Sadow. Outraged, Kressh summoned his remaining allies to rally behind him, and Horak-mul departed the meeting alongside the Sith Lord. After the meeting, Naga Sadow, now officially crowned as the Dark Lord of the Sith, staged a Republic attack on the Starbreaker 12's hangar on Ziost and absconded with the ship. Upon witnessing the scene, Ludo Kressh discovered Sadow's emblem amidst the destruction. Shocked and angered, Kressh summoned Horak-mul and Dor Gal-ram to his private world of Rhelg to discuss these developments. There, Kressh informed them of Sadow's involvement in the assault and ordered Horak-mul and Gal-ram to gather their fleets to launch a surprise attack on Sadow.
Horak-mul followed Kressh's orders, assembling his forces and joining Kressh's and Gal-ram's fleets at Khar Delba, the location of Sadow's citadel. They initiated their attack, unleashing a barrage of fire upon Sadow's base. Horak-mul confidently proclaimed that victory was assured; however, Sadow had a hidden strategy. Sadow had concealed his fleet in orbit around Khar Shian, the moon of Khar Delba, and the fleet launched a surprise attack on Horak-mul and his allies. Sadow then proceeded to the next phase of his plan, ordering the crew of Horak-mul's ship, who were actually Sadow'een—Sadow's elite assassins—to assassinate him. The Massassi crew attacked Horak-mul while he was distracted by the battle, catching him off guard, and murdered him—though Horak-mul would later claim that it took five assassins to kill him. Dor Gal-ram was killed by the same treachery, and Kressh was defeated in the Battle of Khar Delba.
The artificial eye of Horak-mul was eventually lost, but was recovered in 3963 BBY, more than a millennium after Horak-mul's death, by Celeste Morne, a Jedi Covenant Shadow. Horak-mul was later interred on the moon of Yavin 4 by his followers. However, scavengers removed Horak-mul's sarcophagus from the moon over a thousand years later and were transporting it aboard their ship, the Starrunner, when the Sith Lord's spirit drove the crew insane above the planet Hoth. Several years later, in 3642 BBY, Horak-mul's spirit was inhabiting the body of an Ortolan chief when he encountered Kallig, the Sith Inquisitor of the reconstituted Sith Empire. Kallig possessed the power of Force walk, which allowed him to bind Force ghosts to his own body and draw upon their power. Horak-mul agreed to allow Kallig to bind him if the Sith would complete a task for him: the destruction of a Sadow'een Temple on Hoth.
After Kallig desecrated the temple, Horak-mul possessed the shell of a droid to inform Kallig of the Starrunner's location. Upon locating Horak-mul's spirit in the depths of the crashed superdreadnaught Star of Coruscant, Kallig bound Horak-mul's spirit, and the ghost assisted the Inquisitor in the young Sith's battle against the Sith Lord Darth Thanaton. However, Horak-mul and the other Sith spirits whom Kallig had bound later mentally harassed Kallig, threatening to drive the Sith insane. As he was haunting the Inquisitor's thoughts, Horak-mul said: "You think you're the puppeteer, but you're just the puppet."
Eventually, Kallig discovered a mind-healing ritual on Voss that could subdue the ghosts. With help from a Voss dream-walking cult, Kallig confronted Horak-mul in a dream. The Sith Lord assumed the appearance of Darth Thanaton but his illusion was destroyed when a Gormak Force-shaman named Hadrik used a dream-rock to force Horak-mul to reveal himself. Kallig then used Horak-mul's power, along with the other ghosts, to finally defeat Thanaton and assume his seat on the Dark Council. Though the other ghosts pleaded to be freed, Horak-mul willingly decided to stay, knowing that he'd at least see the galaxy rather than be bound to his tomb on Hoth.
Horak-mul was a firm supporter of Ludo Kressh. He viewed Naga Sadow's ambitions to expand the Sith Empire as foolish and detrimental. However, given the popularity of supporting Sadow among Sith Lords and the inherent risks of opposing the Dark Lord of the Sith, Horak-mul was careful to conceal his opposition to Sadow's expansionist goals. Before the attack on the Starbreaker 12, which Sadow manipulated to appear as a Republic raid, Horak-mul considered the Republic to be a myth. However, the attack convinced him otherwise. Horak-mul had an artificial right eye referred to as the Eye of Horak-mul. He also wore a ceremonial robe. During his duel with Kallig on Voss, the spirit of Horak-mul wielded a purple-bladed lightsaber.
Horak-mul's first appearance was in Star Wars: Tales of the Jedi – The Golden Age of the Sith, which is a story arc within the Star Wars: Tales of the Jedi comic series. This story was written by Kevin J. Anderson and released in 1996. In these appearances, Horak-mul was illustrated by Dario Carrasco, Jr.. He was later mentioned in Knights of the Old Republic 25. In 2008, Horak-mul was included in The Complete Star Wars Encyclopedia.
If the Sith Inquisitor chooses to release the ghosts at the end of Act III of Star Wars: The Old Republic, Horak-mul refuses to leave and remains with the Inquisitor. However, if the Inquisitor leaned more towards the light side, they had the option to allow the ghosts to pass on.