When the Goa'uld still held dominion over Earth, Nirrti had little interest whatsoever in political bickering or military power. Her primary concern was her study of humans, and how their physical abilities as hosts could be improved. The other Goa'uld, however, would not give her the time or opportunity to delve into her studies. Kali, in particular, aggressively sought to take over Nirrti's holdings in present-day India. Thus, Nirrti was forced to play the political game she so loathed. She partnered with Olokun, a relationship which would prove disastrous for him, but beneficial to Nirrti. She slowly but surely established a large power-base and by the time of the Tauri rebellion in Egypt, she had more than earned the title of System Lord.
Her continued research on how to modify human hosts led to devastating casualties in her slave populations, and she took to using the slaves of other System Lords - without their permission. Bastet and Kali, infuriated, managed to get Nirrti stripped of her System Lord status. By this time, however, the galactic empires of the Goa'uld were vast, and even while demoted, Nirrti had no trouble finding wayward human populations to perform tests on, and this was all the really mattered to her.
While investigating the Lilin, Doctor Daniel Jackson found information that indicates Nirrti killed their creator, the Ancient Lilith. While Nirrti is stated to be one of Lilith's children in the information Daniel found, he thinks it more likely Nirrti was Lilth's protégé or had taken her as a host briefly to get access to her knowledge on genetics before killing her. (SGA: "Blood Ties")
Nirrti seemed obsessed with creating enhanced humans (hok'tar) which could serve as hosts. She tried various experiments including killing the inhabitants of the planet Hanka with a bacterial infection, the only survivor being a young girl named Cassandra in whom she planted a bomb in an effort to destroy Earth's Stargate. Fortunately, Cassandra lived and was adopted by Dr. Janet Fraiser, the SGC's Chief Medical Officer. (SG1: "Singularity")
Later, when the System Lords agreed to negotiate with the Asgard to include Earth in the Protected Planets Treaty, Nirrti accompanied Cronus and Yu as part of the Goa'uld delegation. During the negotiations she was adamant that the Tau'ri were too powerful and aggressive toward the System Lords for the Goa'uld to allow them to go unchecked, essentially arguing the Tau'ri were fair game and did not need the protection of the Asgard. Also during these discussions it was revealed that Nirrti had long aspired to capture Cronus' domain. Using cloaking technology, Nirrti attacked Cronus in his quarters, framing Teal'c for the attack. Colonel Jack O'Neill decided to bluff, saying that they knew she was experimenting with phase shifting technology, and she exposed herself. She was captured and became a prisoner of Cronus. (SG1: "Fair Game")
After Cronus was killed by SG-1 two years later, Nirrti escaped his prison and managed to rebuild her resources while the rest of the System Lords were distracted by the return of Anubis. She found out that Cassandra Fraiser was still alive and attempted to find more information about Cassandra in order to conduct more experiments on other people. At one point, she managed to sneak into Stargate Command using a Goa'uld cloaking device that made her invisible, having originally been waiting in her own lab for the chance to do so. When she was eventually discovered and captured, she offered to help save Cassandra, who was dying of a retrovirus that was turning her into a hok'tar. Nirrti demand her freedom, cloaking device, and a sample of Cassandra's blood in exchange. Major General George S. Hammond initially refused. In an act of desperation, Dr. Janet Fraiser held Nirrti at gun point, demanding that she help. After Hammond agreed to free her, and reminded her that Fraiser was Cassandra's adopted mother, Nirrti healed the girl, and was allowed to leave unharmed. Although Nirrti admitted that without a sample Cassandra's blood she was forced to start all over with her research. (SG1: "Rite of Passage")
During a talk concerning the upcoming System Lord Summit, Major Samantha Carter expressed some concerns about Nirrti showing up at the Summit. Sam's father, Jacob Carter revealed that there wasn't any chance of that happening, given how Nirrti was still a renegade and also "persona non grata" among the System Lords. (SG1: "Summit")
K'tano's rebel Jaffa captured a shipment of weapons intended for Nirrti's Jaffa on a planet in her domain. K'tano managed to convert many of her Jaffa to his cause. He later sent a team of rebel Jaffa on a suicide mission to another planet in Nirrti's territory. (SG1: "The Warrior")
Adrian Conrad's Goa'uld was able to help Colonel Frank Simmons make a deal with Nirrti, allowing her to capture Dr. Janet Frasier as part of Nirrti's revenge for defying her attempts to analyze Cassandra Fraiser. In exchange, Nirrti gave Simmons information on how to genetically engineer humans into Jaffa. Her plan was thwarted when SG-1 managed to rescue Frasier, with Major General George S. Hammond and Bra'tac leading the effort to kill and/or capture the 'new' Jaffa that she had created. (SG1: "Survival of the Fittest")
The Russian SG-4 team later discovered her experiments to create a hok'tar on the planet P3X-367, this time through genetic manipulation, finding Alebran in his ruined city after he had left Nirrti. They brought him back to Stargate Command to speak to SG-1 as it was because of them that Nirrti was still free. Alebran informed them of all that he knew but soon died due to Nirrti's experiments, seeming to turn into liquid.
The two SG teams returned to the planet in order to help the natives, but the natives did not believe that Nirrti was evil as she had been the one to save them from a plague which had ravished their planet. Teal'c and Jonas Quinn were taken prisoner by the natives after Eggar discovered their intent to kill Nirrti in O'Neill's mind, and the others were soon captured upon Nirrti's return also.
SG-1 attempted to inform the native's of Nirrti's true motives while Nirrti performed experiments on both teams, meeting with no success for some time. Nirrti performed several experiments on members of both teams before finding that Jonas' Kelownan heritage had caused him to be somewhat more evolved than the Tau'ri. She was sure she could create her hok'tar in him, and even asked if he would submit to allow her to do so. Although Jonas rejected the offer, Nirrti nevertheless performed several experiments on him. During this time, however, SG-1 was able to somewhat convince the natives of Nirrti's true intents and after Eggar read her mind, he discovered this to be true.
Wodan then killed Nirrti by telekinetically breaking her neck, but not before she made subtle modifications to Jonas's DNA that resulted in him developing the ability to see into multiple futures during a later mission. Her efforts were short-lived as Eggar had read her mind before her death, allowing him to reverse what she'd done to everyone including Carter.(SG1: "Metamorphosis")
Unknown to anyone at the time, Nirrti's experiments on Jonas Quinn gave him the ability of precognition through a brain tumor that formed. Jonas had only limited control over it and unlike what Nirrti believed, his Kelownan heritage did not allow his body to stabilize, though he didn't liquefy. Ultimately, the tumor was removed and Jonas lost the ability Nirrti gave him, but not before he used it to aid SG-1 in defeating the Goa'uld Underlord Mot and his Jaffa who were ruling P4S-237 and tried to attack Earth. (SG1: "Prophecy")
- In an alternate timeline created by Ba'al, he conquered several System Lords, including Nirrti, who served under him and participated in an attack on Earth. She, along with Qetesh, believed that the Tau'ri "should be blasted to the brink of extinction", finding such an idea "enjoyable". (SG1: "Continuum")
- When explaining Nirrti's history on Hanka, Teal'c refers to Nirrti as him instead of her. This could be a mistake as the episode aired over a year and a half before Nirrti formally appeared on the show or she could've had a male host at one time.