Darth Nihl, a male Nagai hailing from the planet Nagi in the Unknown Regions, served as a warlord on his homeworld. He utilized his inherent Force abilities to command bands of marauders in raids across the world. Darth Krayt, who was in the process of rebuilding the Sith Order, discovered him and extended an invitation to join the One Sith. Nihl accepted and underwent training to become a Sith Lord. After his initial experience with the Embrace of Pain, his rage and suffering drove him to slaughter an entire village. He was known as Krayt's primary enforcer within Krayt's Sith Order, serving the Dark Lord as his Fist, or military commander. Around 130 ABY, Darth Krayt promoted Nihl to the position of Hand after his predecessor perished during the war, in which the Sith aligned themselves with the Fel Empire to overthrow the existing galactic government, the Galactic Alliance.

Two standard days after the Empire seized Coruscant, the Galactic Alliance's capital, Nihl spearheaded the Sith in an assault on the Jedi Temple on Ossus, under Krayt's orders to eliminate all Jedi. However, while executing his Lord's command, Nihl and his Sith and stormtrooper forces were confronted by Kol Skywalker, a Jedi Master from the New Jedi Order, who sought to provide a group of Padawans with an opportunity to escape Ossus. Despite some initial success against the Sith by Skywalker and his former Padawan, Zabrak Wolf Sazen, their efforts proved futile. Nihl engaged Sazen in combat, utilizing his long-handle lightsaber to defeat the Jedi Master. Sazen's weapon arm was severed, and he was subsequently impaled by the Sith Lord, resulting in a mortal wound. Following Sazen's defeat, Skywalker directly challenged Nihl alone, injuring him in a final stand.

Skywalker's son, Cade, soon arrived to aid his father in combat, who had been momentarily distracted from his duel with Nihl. The elder Skywalker instructed Cade to escort the wounded Sazen to the Jedi's escape shuttle, preventing Nihl from killing the injured Master. Skywalker continued his resistance against Nihl and his Sith, but to no avail; Nihl unleashed his Force lightning to kill the Jedi Master. With the majority of the Jedi on Ossus dead, Nihl departed the planet, having triumphed over the New Jedi Order. He journeyed to Coruscant, where he witnessed Krayt's ascension as Emperor of a newly established Sith Empire by assassinating a decoy of Emperor Roan Fel to seize control of the Fel Empire. Following the decoy's death, the Imperial Moffs pledged their allegiance to Krayt.

Seven years later, Nihl no longer held the sole position of Krayt's Hand. Darth Talon, a Twi'lek Sith, was appointed as Krayt's second Hand and dispatched to Socorro for her initial mission: to capture and utilize Princess Marasiah Fel to lure her father, the former Emperor Roan Fel, into the open, enabling Talon to eliminate him and quell any resistance to Krayt's complete dominion over the galaxy. However, Cade Skywalker, who had turned to bounty hunting after Nihl killed his father, intervened and prevented Talon from capturing Marasiah. Skywalker and his companions, the Zeltron Engineer Deliah Blue and Human co-pilot Jariah Syn, transported Marasiah and her aide, Astraal Vao, to Vendaxa to meet with Vao's brother, Jedi Knight Shado, and Wolf Sazen, the Jedi Nihl had presumed dead. Talon attacked them there, albeit unsuccessfully, and requested the Nagai's assistance in eliminating the Jedi and capturing the princess. Talon instructed Nihl to discreetly deploy troops to her location, await the former Emperor's arrival, and then ambush the Jedi and Imperials to kill them all.

Fel arrived with an escort of two Imperial fighters, which were quickly shot down by the Sith troops positioned by Nihl and his fellow Hand. However, Fel and his accompanying Imperial Knights escaped the destroyed ships only to be confronted by Talon. At that moment, Fel revealed himself to be Imperial Knight Antares Draco. Confused by the deception, Talon was forced to retreat and regroup with Nihl and his troops. He planned to attack the Jedi and Imperials once more, hoping to capture Marasiah and kill the others. As they attacked, Nihl again dueled with Sazen, mocking the Jedi Master with the prospect of killing him a second time. Skywalker, seeing the Nagai Sith dueling Sazen once more, and Marasiah mortally wounded by a Zabrak Sith, joined Sazen in combat, hoping to avenge his father. Sazen attempted to halt Skywalker's attack on Nihl, who believed that Sazen was right to stop Skywalker, and stated that wielding a lightsaber did not make him a Jedi. The Sith Lord further mocked him and explained that vengeance was not a Jedi trait, though Skywalker did not care, and continued his duel with his father's killer.

As Nihl's duel with Skywalker continued, Sazen was sent to bring the princess onto Cade's ship, the Mynock. Nihl, convinced that the bounty hunter could not defeat him, offered him a quick death similar to his father's, but Skywalker simply pressed on, discontent with any result besides the Nagai's death. Nihl was subsequently shot by Syn, allowing Skywalker enough time to gather himself, but his companion was soon Force pushed away by Nihl. This left him open to defeat by Skywalker, who instead escaped with his companions, the Imperials, and Jedi Vao, on the Mynock. Nihl then explained to Talon that he would soon be the only Hand once again, as he believed Talon would be killed by Krayt for the mission's failure.

To Nihl's disappointment, Talon was not killed for her actions on Vendaxa. Instead, a search was warranted for Skywalker, in whom Krayt had an interest because of his potential to heal through the Force. This search, once begun, was easier than expected; Skywalker came to the Sith Temple on Coruscant to save Hosk Trey'lis, a Bothan Jedi Knight whom Skywalker had handed to the Sith for a bounty before experiencing a change of heart. He was attacked by Talon, whom he defeated using the dark side of the Force. Before he could finish her off, Nihl Force choked him with the intent of killing the last Skywalker. However, Talon informed Nihl that Skywalker was wanted alive, and that she was sent to capture him. Unable to disobey Krayt's orders, Nihl obliged, and informed Krayt of Skywalker's capture.

With Skywalker captured, Krayt hoped to turn the former Jedi to the dark side. By threatening to kill Syn and Blue, the Sith Lord was able to persuade Skywalker to listen to his offer and reconsider being a Sith. Krayt brought Skywalker to his chambers, but when Nihl attempted to enter, Darth Wyyrlok, Krayt's chief servant, barred him from entering. Nihl attempted to argue, believing his status as a Hand was enough. Wyyrlok did not comply, and questioned Nihl's loyalty to Krayt, causing Nihl to step back. Darth Maladi, the head of Sith Intelligence and Assassination, explained to Nihl why he wasn't allowed entry; Nihl was not born a Sith. Krayt had wanted Nihl to become his Sith military leader, and only promoted Nihl to Hand because Talon had not been ready at the time of his predecessor's death. When Nihl questioned Maladi as to why she wasn't allowed to enter even though she was born Sith, she stated her complete loyalty to Krayt. However, both she and Nihl wondered at Krayt's fascination with Skywalker.

Krayt's attention to Skywalker paid off, and Skywalker agreed to become a Sith, trained by Talon. As Skywalker fell further and further down the dark side, Krayt became convinced that Skywalker would be able to heal him of the Yuuzhan Vong Yorik-Kul implants that were causing his body to fail. As Skywalker began to realize his power under the training of Krayt and Talon, the Sith Lord became obsessed with Skywalker and his potential ability to heal him. This obsession Nihl disagreed with, and he told Maladi of this. She began to wonder to herself if Skywalker was capable of healing Krayt, and what would happen should he die. Nihl firmly believed in the old ways of the Sith; because Krayt had named no heir, there would be a power struggle. Therefore, Nihl believed that should Krayt fail to be healed, then Skywalker would be named his heir.

However, Skywalker had different plans; he lied about his eagerness to be trained as a Sith, and attempted to escape the Temple of the Sith. This escape attempt was foiled by Talon, and Skywalker was brought to Krayt to kill Trey'lis, so that he could become a Sith in truth. Nihl watched as Krayt commanded him to kill Trey'lis or be killed himself, but Skywalker refused. Krayt attempted to stir Skywalker's hatred of people dying for him as motivation to kill Trey'lis, but the Bothan Jedi intervened, expressing his hope that he would not be used as a weapon of the Sith. For his interference, Krayt struck him down and a skirmish between Nihl, Talon, Krayt, and Skywalker occurred when the latter had a vision of his father. Using the Force, Skywalker pulled his father's lightsaber toward him, impaling Talon with it, and engaged Nihl in combat.

As Nihl defended himself against Skywalker, he was betrayed by Krayt; the Dark Lord encouraged Nihl's opponent to use his anger and kill the Nagai in vengeance. The duel continued, and eventually Nihl's arm was severed by Skywalker. Defeated and unconscious, he was then attacked by Skywalker using Force lightning, whose intention was to kill Nihl as revenge for his father's death. However, upon Krayt's beckoning, Skywalker engaged the leader of the Sith in combat instead of killing Nihl. Skywalker was subsequently rescued by his mother, Morrigan Corde, who had aligned herself with her son's companions in a plan to rescue the bounty hunter. The plan was a success and Skywalker escaped, resulting in the failure of both Nihl and his fellow Hand to keep Skywalker within the clutches of the Sith. Krayt was determined to make them both pay for their failure.

Immediately after Skywalker's escape, Nihl was tended to by Maladi and Wyyrlok, along with Talon and Krayt. Though the latter was not injured, Nihl and his fellow Hand were hurt extensively, and Maladi was put in charge of their healing. Since the Nagai's arm was cleanly cut, Maladi offered to have it reattached, but Krayt refused; Nihl's incompetence had failed the Sith, and Krayt felt that he was no longer worthy of his arm as a result. Instead, Nihl was to receive a Yuuzhan Vong bioform replacement for his severed limb. Krayt had originally wanted to kill Nihl for his failure, but Wyyrlok convinced him otherwise, explaining that while he was no longer able to continue his position as a Hand, Nihl still had use to the Dark Lord. Krayt then replaced Nihl with Darth Stryfe, his Fist, as his new Hand, and sent him and Talon to be healed. Nihl protested his replacement, saying he had earned his place as Krayt's Hand, but Krayt brushed the complaint off. Maladi took the two to her laboratory, and placed both Nihl and Talon in bacta tanks.

As the injuries he sustained were more severe than Talon's, Nihl continued to heal on Coruscant, while the Twi'lek recovered completely. The former Hand remained in bacta while Lords Krayt, Maladi, Stryfe, and Talon answered the summons of a mysterious female Jedi, who claimed to have Cade Skywalker imprisoned on the Deep Core world of Had Abbadon. Lured into a trap by Skywalker and his colleagues, which included several Imperial Knights, Nihl's fellow Sith were attacked and subsequently defeated. Though Darth Krayt survived the battle, he was murdered shortly afterward by Darth Wyyrlok, who feigned the Dark Lord's survival to all while acting in Krayt's stead as both leader of their Order and ruler of the Empire. Meanwhile, Nihl had recovered fully and bore a new arm, and later joined Wyyrlok with several of the more prominent Sith when they returned to Coruscant to secure the Imperial throne in the name of Darth Krayt.

Despite the fact that Lord Wyyrlok had successfully misled the majority of Sith Lords, including Darth Stryfe, and the Moffs into the false belief that Emperor Krayt had survived the assassination attempt and entered prolonged stasis for recuperation, Darth Maladi, a master of deception herself, harbored skepticism. On Wayland, Nihl convened with Maladi, where she voiced her apprehension that the Chagrian Sith Lord would strive to seize control, likely by eliminating those he deemed a threat; Lady Maladi surmised that both she and Nihl, as individuals not born into the Sith, would be targeted for removal. She proceeded to assert that she regarded him as a significantly more suitable leader for the Sith and persuaded him to journey to Korriban to substantiate her suspicions regarding Krayt's demise and subsequently utilize his findings as he deemed appropriate. Nihl proceeded to the Temple on Korriban, where he encountered and was momentarily impeded at the entrance of the Stasis Room by Darth Talon. Upon her refusal to grant him access, Nihl fabricated a story, claiming that Darth Krayt himself had summoned him. They entered the room together, and Talon was taken aback to discover that Lord Krayt's physical body was absent, leaving behind only his vacant armor, a scenario for which Maladi had prepared Nihl. Shortly thereafter, Lord Nihl contacted Darth Wyyrlok's office via a high-priority channel. He promptly informed Wyyrlok of Lord Krayt's disappearance.

Wyyrlok's arrival on Korriban occurred soon after, and alongside Nihl, he scrutinized the stasis chamber where Krayt was purportedly residing. As Nihl listened, Wyyrlok asserted that an individual within the Order must have murdered Krayt and absconded with his remains. Wyyrlok further elaborated that he and Nihl were obligated to maintain the Dark Lord's death a secret while Wyyrlok governed the Sith until a successor could be designated; Nihl himself was selected to serve as Wyyrlok's advisor. Despite his prior knowledge of Krayt's death and the identity of the true perpetrator, Nihl swiftly proposed that Darth Talon, his longstanding adversary, might bear responsibility, given that she had not only entered the tomb alongside Nihl but had also mysteriously vanished following the discovery. Delighted to have identified a scapegoat, the Chagrian concurred and instructed Nihl to locate and interrogate Talon while he returned to Coruscant to address matters concerning the Empire. Nihl initiated his search throughout the Valley of the Dark Lords, where he established a bond with a Tuk'ata to assist him in the pursuit.

Nihl located Darth Talon within the tombs on Korriban, and as they engaged in a lightsaber duel, he implicated her in the assassination of Darth Krayt. Much to Nihl's astonishment, Lord Krayt materialized suddenly, appearing alive, healthy, and more potent than ever before. Nihl was reprimanded for his ambition and aspiration for power at Krayt's expense, but the Dark Lord opted to spare his life regardless. Alongside Darth Talon, Nihl accompanied Darth Krayt to a clandestine chamber where, for numerous years, he had been amassing an arsenal of Force-sensitive Sith troopers along with new Sith fighters for his burgeoning army, an army with which he intended to overwhelm the opposition and forcefully enforce peace and order throughout the galaxy.

Around 138 ABY, Nihl went with Darth Krayt as the Dark Lord prepared to take back his throne from Darth Wyyrlok, who had taken it without permission. They went to Coruscant on one of Krayt's new Dragon ships and attacked the Sith temple when they got there. Darth Nihl, Talon, and two of Krayt's Sith troopers fought through many of the Sith who were loyal to Darth Wyyrlok as they went to the throne room, and the Chagrian met them at the entrance to fight. No one else got involved in the fight between him and Darth Krayt, and when Krayt won, Nihl went back to being by the Dark Lord's side. Soon after, Lord Krayt found out where the hidden Jedi Temple was on Taivas, and got his forces ready to fight his enemies. Darth Nihl was told to work with the Moffs as the Imperial fleet got ready to attack Taivas, and was told to keep them from knowing about Krayt's Sith troopers, who were secretly under his control. During the Sith ambush, the Dark Lord set his trap and told Nihl to let the troopers go, a confusing group that killed everyone in their way, no matter who they were loyal to, as they attacked the Hidden Temple. After they won on Taivas, Nihl and his Sith troopers hurried to meet the allied forces in battle for Coruscant.

However, the Sith did not think the enemy was as strong and determined as they were, and they were defeated on all fronts. In the middle of the Sith Temple, Darth Krayt was killed in a fight by Cade Skywalker, who had just become a Jedi, and people who were connected to the dark side felt death shudders all over the galaxy. Darth Nihl then told the One Sith to retreat right away. The Sith troopers, who were connected to Darth Krayt's life, were driven crazy by his death, and the Nagai Sith Lord ordered them to be killed. The Sith fleet was destroyed, Korriban was evacuated by any survivors, and Sith on battlefronts all over the galaxy retreated to places they had planned to go to, to gather their remaining power. Nihl was joined by the Inquisitor Darth Havok, and he told him about the new way the exiled Sith would work: they would be patient, cunning, and secretive; they would infiltrate galactic governments to destroy the foundation of the galaxy, and create it again.

Darth Nihl started out as a warlord on his home planet of Nagi, and later joined Darth Krayt in his new Sith Order. As a member of the One Sith, Nihl followed Krayt's orders without question, but sometimes he was not sure about the decisions the Sith Lord made; for example, Nihl did not agree with Krayt's focus on Skywalker, or his desire to make the former Jedi a Sith. Nihl had studied the old ways of the Sith, which said that the strongest Sith should be in charge of the Order, not an heir. Nihl thought Skywalker was going to be Krayt's heir, and wondered why Skywalker was getting privileges that Nihl had not been given, such as being allowed into Krayt's chambers, when Nihl had already proven that he was loyal to Krayt.

Before Krayt became obsessed with Skywalker, Nihl had taken a personal interest in him. Nihl had killed his father at Ossus, and Skywalker's desire to get revenge on Nihl made him notice the former bounty hunter. Nihl was also ambitious; he wanted to be the only Hand, and hoped that Krayt would kill Darth Talon, his companion as a Hand, for her failures. The Nagai had to take orders from Talon, such as when she ordered the confrontation with Fel on Vendaxa, and when she stopped Nihl from killing Skywalker, which Nihl did not agree with. Darth Maladi later used Nihl's ambition to get him to join her against Darth Wyyrlok, but she did not understand him because he joined Wyyrlok when he promoted him to keep him loyal. Nihl used his new position to go against his longtime rival, Darth Talon, which was what Maladi was afraid Wyyrlok would do with her and Nihl because they were not born Sith. Nihl also thought that his own abilities as a Sith made him better than others. He made fun of Sazen's talent as a Jedi because the Sith had cut off his arm with his lightsaber, and made fun of how much Skywalker had been taught as a Jedi, believing that just because Skywalker could use a lightsaber, it did not make him a Jedi.

Darth Nihl, as a Sith, had knowledge of basic Force skills such as telekinesis. He used Force push and pull against Jariah Syn in a fight on Vendaxa. Nihl also knew the Sith skill of Force lightning, which he used to kill Kol Skywalker on Ossus. These skills, along with Nihl's use of a long-handle lightsaber, showed that he was a skilled duelist; he was able to kill a member of the Skywalker family, Jedi Master Kol Skywalker, and was able to defeat Wolf Sazen, a Jedi Master, in combat. Another skill Nihl used was Force choke, which he used to subdue and capture Cade Skywalker when he went into the Temple of the Sith. However, Nihl could not use his skills in dueling to defeat Skywalker in a second duel when he tried to escape the Sith temple, in which Nihl was defeated and lost his arm. Nihl also used his skills with a lightsaber in his duel against Darth Talon right before Darth Krayt came back to life. As he chased Darth Talon in the Valley of the Dark Lords, Darth Nihl showed that he could tame a Tuk'ata.

Darth Nihl's first appearance was in the Legacy series of Star Wars comic books, where he was an important character and enemy in the story. He was first mentioned briefly in the pre-release issue of Legacy, 0, Nihl appeared often in the Broken story arc, where he killed Cade Skywalker's father Kol, which made Cade want to get revenge for his father's death, and caused Cade to fall to the dark side. Nihl then appeared briefly in Allies, where he had a small role, and did not appear again until the next major story arc, Claws of the Dragon, where Nihl captured Skywalker, but was defeated when Skywalker escaped the Sith. Since then, Nihl has appeared in Legacy (2006) 23, and Legacy (2006) 27 which talked about Nihl being replaced as Hand by Darth Stryfe for failing, and later in the Monster story arc with Darth Maladi. Nihl was also listed in Lucasfilm official Leland Chee's blog, where it was revealed that his name comes from the words "nill, and nihilist." Because Nihl was an important character for the first nineteen issues of Legacy, he was mentioned in several sourcebooks, ads, and action figures, including The Complete Star Wars Encyclopedia, Jedi vs. Sith: The Essential Guide to the Force, a miniature set for Legacy of the Force, and an ad for the miniature set.