The conflict known as the Battle on Had Abbadon unfolded in 137 ABY. This clash was orchestrated by a coalition of Jedi and Imperial Knights, who devised a scheme to eliminate Darth Krayt, the Dark Lord of the Sith and the current Galactic Emperor. The battle erupted when Krayt's Imperial garrison on Had Abbadon faced an attack by the allied Knights and Jedi. During the assault, Darth Reave, a servant of Krayt, was deliberately allowed to escape to inform Krayt that Cade Skywalker, a Jedi, was among the attackers. Simultaneously, Karness Muur, whose spirit possessed Jedi Master Celeste Morne, a companion of Skywalker, also contacted Krayt. Muur proposed to heal Krayt from the Yuuzhan Vong implants that were killing him, offering a partnership in ruling the galactic empire, but only if the Dark Lord ventured alone to a world within the Deep Core. Ignoring Muur's instructions, Krayt arrived on Had Abbadon accompanied by Darths Maladi, Stryfe, and Talon, while his trusted lieutenant Darth Wyyrlok remained aboard their ship. Despite Krayt's disobedience, Muur proceeded to demonstrate his healing abilities. It was at this point that the Jedi and Imperials sprung their trap, engaging Darth Krayt and his Sith Lords in combat.
Morne, using her control, unleashed her rakghoul mutants to attack while she personally dueled Krayt. Imperial Master Azlyn Rae saw an opening and struck down the Dark Lord, mortally wounding him. Morne then unleashed a deadly surge of Force lightning upon Krayt, inadvertently engulfing Rae in the process. The battle appeared to conclude with Krayt's apparent demise and Celeste Morne's death at the hands of Skywalker, who granted her request to be freed from Muur's spirit. The Jedi and Imperials then gathered their wounded and retreated from Had Abbadon. Meanwhile, Darth Wyyrlok landed a shuttle to retrieve his defeated Sith comrades, discovering Darth Krayt still alive. When the Dark Lord requested medical attention from Wyyrlok, the Chagrian instead betrayed and seemingly killed him. However, Krayt, having sustained severe injuries from Wyyrlok's attack, was resurrected and soon revealed his survival to the treacherous Sith.
By 137 ABY, Darth Krayt was under the impression that his end was near, and that he would not live to see his vision of a stable galaxy under Sith governance realized. Despite his relentless efforts, he remained unable to rid his body of the Yuuzhan Vong biots that threatened to consume him. Krayt had received information from one of his minions that Cade Skywalker, a Jedi possessing remarkable healing powers, had engaged and defeated Krayt's garrison on the Deep Core planet of Had Abbadon. Furthermore, Krayt had been summoned by an ancient Sith Lord named Karness Muur, whose spirit inhabited a female Jedi Master named Celeste Morne. Muur claimed to hold Skywalker captive on Had Abbadon and offered Krayt both Skywalker as a prisoner and a chance to be healed by Muur's own power—should Krayt allow Muur's spirit to inhabit his body instead. Muur instructed Krayt to come alone to Had Abbadon, threatening to kill both Skywalker and Krayt if he failed to comply.
In reality, Skywalker and Morne were attempting to draw Darth Krayt to the Deep Core to ambush and assassinate him. They joined forces with Twi'lek Jedi Shado Vao and Imperial Knights Antares Draco, Ganner Krieg, and Azlyn Rae to await the arrival of the Dark Lord of the Sith. All participants hid their presences in the Force, anticipating the enemy's arrival.
Defying Muur's explicit instructions, Krayt landed on Had Abbadon with Darths Maladi, Stryfe, and Talon. His chief advisor, Wyyrlok, was ordered to remain in orbit to gather more intelligence on Muur and his purported powers. Muur appeared before Krayt in Morne's body, with Skywalker seemingly held captive. Muur rebuked Krayt for his disobedience, reminding him of their agreement; however, when Krayt questioned the spirit's healing capabilities, Muur briefly demonstrated them. At that moment, Skywalker summoned his lightsaber while breaking free from his bonds, attacking Darth Talon, who had sensed his intentions at the last possible moment. Imperials Draco, Krieg, and Rae, along with Jedi Shado Vao, initiated their ambush, engaging the Sith in battle.
Through Muur's mastery of Sith magic, Morne directed her army of Sithspawn-rakghouls to attack Krayt, while the other Lords dueled with the Jedi. Darth Stryfe used Force lightning to eliminate the rakghouls surrounding Krayt, while Darth Maladi engaged Shado Vao. Openly expressing respect for her opponents' Sith-influenced strategy, she assaulted the Twi'lek with her own lightning barrage. Despite the intensity of her attacks, Darth Maladi remained evenly matched with Vao.
After swiftly defeating Darth Talon, Skywalker attacked Stryfe, Force-pushing the Sith Lord into a pile of thermal detonators set by Skywalker's friend Jariah Syn. The resulting explosion engulfed Stryfe, throwing him off the platform. Skywalker then turned his attention to Morne's rakghouls, who were wreaking havoc and terrorizing Jedi, Sith, and Imperial Knight alike indiscriminately.
Amidst the fight against the Sithspawn, the Imperial Knights retreated to assess their role in the conflict. Draco believed it was their duty to retrieve Morne's Sith talisman, which granted her the ability to transform non Force-sensitive beings into rakghouls. However, Rae argued that destroying Krayt was paramount, advocating for a unified effort towards that goal. Draco responded by Force-pushing Rae to the ground in disagreement, prompting Krieg to intervene. Draco then attempted to approach Morne but was immediately detected and repelled with the Force. Muur's spirit seized control of Morne, exposing Draco's treacherous motives while vowing to kill the Jedi, transform Skywalker's companions into rakghouls, and claim Darth Krayt's body as its own.
At the critical moment, Krayt unleashed a massive blast of Force lightning, repelling all the rakghouls and temporarily halting the battle. He then confronted Morne, coaxing her to embrace the dark side power of Muur that Krayt sensed within her. Abandoning their lightsabers, Krayt and Morne exchanged volleys of Force lightning. Maladi joined the Dark Lord, adding her power to his in an attempt to overwhelm Morne.
Seeing that Krayt was occupied, Azlyn Rae requested Skywalker to provide cover while she attempted to eliminate Krayt. Skywalker fended off the advancing rakghouls as Rae launched herself at Krayt from behind, successfully stabbing him through the chest with her lightsaber. Krayt's concentration faltered, allowing Muur to unleash a devastating torrent of lightning that consumed both Rae and Krayt, scattering the remaining combatants across the platform. Muur then hurled Krayt's defeated body over a nearby cliff to his apparent death. Rae suffered severe burns over most of her body, with nearly all her hair singed away.
Following the battle, the Sith were gone, and the rakghouls had been eradicated. Morne momentarily regained control of herself from Muur before confronting Skywalker. Aware that Muur would soon take permanent control, Morne accepted her fate and pleaded with Skywalker to kill her. He complied, running her through with his blade. Upon her death, Morne's body turned to dust.
The amulet of Muur's in Morne's possession that both housed his spirit and power, now free from Morne's body, immediately latched onto Skywalker's arm. Muur believed that he would make this Jedi his new thrall; instead, Cade displayed his strength of will by resisting Muur's temptation. He poured the Force into the weak points of the talisman and caused it to shatter, which in turn caused Muur's spirit to fade.
Cade then turned his attention to Azlyn's condition and moved to take her aboard the Mynock to get help. He was stopped by Ganner who claimed that as an Imperial Knight, Rae belonged with them. However, Vao used a Force push to knock Ganner and Draco away and he, Cade, and the rest of Cade's crew left to seek help from Bantha Rawk.
Darth Wyyrlok, having sensed his master in need, had journeyed to the planet surface. There at the bottom of a canyon, he came across Krayt's broken and burned body. Wyyrlok was shocked to discover that Krayt was still alive, having used the Force to cushion his fall. Krayt ordered Wyyrlok to take him to a bacta tank as his encounter with Muur had taught him the knowledge he needed to finally heal himself of the coral growths on his body. However, Krayt would, it seemed, never get a chance to use this knowledge. Claiming that sometimes for a cause to continue, the leader of that cause must die, Wyyrlok killed Krayt with Force lightning before taking his body to be buried on Korriban. However, Krayt was able to resurrect himself through the use of implants, and on Korriban, he once more revealed himself to the mutinous Sith. The next year, Krayt led an attack on the Temple of the Sith on Coruscant and killed Wyyrlok in retaliation for his betrayal.