A Human male Sith Lord known as Grathan was a servant of the reconstituted Sith Empire. During the Cold War era, he opposed the Galactic Republic. In defiance of the Dark Council, he established a clandestine operational hub on Dromund Kaas. Ultimately, his aspirations were severely thwarted by the apprentice of Darth Baras.
During the Cold War, Grathan turned against his superiors, proclaiming himself as the thirteenth member of the Dark Council. He carved out his own sphere of influence and power base within the Sith hierarchy. Though his assertion was not widely accepted, he commanded a considerable private army stationed on Dromund Kaas. In response, the Dark Council sanctioned a full-scale war against Grathan and all who supported him.
After being forced to flee the Citadel, Lord Grathan sought refuge in his estate, a heavily fortified location with experimental shielding situated outside Kaas City. His forces there included loyal Imperial troops, cybernetic supersoldiers developed in his clandestine weapons laboratories, and apprentices. His group of elite scientists – some coerced, others willingly serving – produced a seemingly inexhaustible supply of war droids, practically eliminating attrition within Grathan's military. Vereta Fraabaal was among his apprentices. To demonstrate Grathan's vulnerability, Darth Baras tasked his apprentice with infiltrating his estate and assassinating his son. Grathan's wife attempted to trade her husband's life for their son's, but Baras' apprentice ultimately killed both the mother and the son.
At one juncture, Grathan obtained possession of the mask belonging to Revan, storing it within the depths of his vaults. He subsequently lost it when the Order of Revan orchestrated a raid on his estate, successfully recovering the mask. Eventually, the mask was returned to Revan himself.
One of Grathan's many plots involved an attempt to transmit messages to Tytonus, a Sith Lord in exile who had previously tried to seize an Imperial world. These messages contained schematics of Kaas City, which would have allowed Tytonus to launch an invasion using a small army of mercenaries and a core of Sith adepts trained by herself. The deployment of most of the Imperial army across the galaxy due to the Empire's overexpansion during the prior war created a serious risk that she might have succeeded in seizing the capital, which would have been a disaster for the Empire. However, an Imperial strike team destroyed her fleet of shuttles and killed her, stopping her invasion and Grathan's attempt to destabilize the Dark Council. Major Shellaster, who organized the strike team, noted that although nobody could prove Grathan was behind the transmissions or the invasions, it didn't matter--anyone who mattered knew, and as a consequence Grathan was already losing what little support he had left.
Lord Grathan made his debut as an antagonist in the video game Star Wars: The Old Republic, specifically during the Sith Warrior storyline on Dromund Kaas. According to game files, Grathan was initially named "Lord Rathari", with his wife and son being Cellvanta Rathari and Beelzlit Rathari, respectively. However, the name "Rathari" was later assigned to a different Sith Lord on Nar Shaddaa. A generic character model labeled "Lord Grathan," with the fully-qualified name of "npc.location.companion.sith_warrior.jaesa.lord_grathan", is also present, suggesting that Grathan was initially intended to be involved in the companion missions for Jaesa Willsaam.
The player's choices determine Lord Grathan's ultimate fate. Players have the option to kill his wife and son (a Dark Side decision) or spare their lives and kill Lord Grathan himself, allowing his son to secretly assume his position (a Light Side decision).