Part Two: Project Wideawake & Days of Future Past
Despite the beliefs to the contrary, the Nimrod unit had not been destroyed. While the Omega Sentinels, the predecessor units to Nimrod, had been designed for adaptation, the Nimrod class Sentinels were to take this to a whole new level with instantaneous adaptation. In the moments between the temporal displacement of Rachel Summers and the detonation of the nuclear bomb, the Nimrod unit had surpassed the expectations of its designers by mechanically replicating the procedure. [Uncanny X-Men #193]
Unfortunately for Nimrod, it had not been able to focus its temporal jaunt and found itself caught in an unusual energy flux for which its memory cells had no analogue or explanation. Caught in the space/time continuum, it was lost. However, an unknown force reached out and plucked him from the void and brought it back to Earth, late in the 20th century, ironically close to his quarry.
Rachel Summers had indeed succeeded in her involuntary jump to the past. Some months after her arrival, the ancient sorcerer known as Kulan Gath had arisen from his own oblivion. For time untold he had been imprisoned in a golden necklace which recently had been discovered by a fisherman named Jaime Rodriguez. Having sensed that the item harbored an evil and malevolent force, Jaime swore to get rid of it. However, before he could do so, he was murdered by a mugger, whose touch revived Kulan Gath and ultimately transformed the isle of Manhattan into Kulan Gath’s playground.
Still, a few heroes fought free of Kulan Gath’s subsequent transformation and enlisted the aid of Doctor Strange, who weaved a spell to undo that which Kulan Gath had wrought. 
The subsequent outcome of this, which Strange admitted he would be unable to predict, was the summoning of the lost Nimrod unit, which appeared in the subway just prior to Jaime’s murder. Fulfilling one of its directives to prevent crime, it obliterated the mugger and saved Jaime.
Immediately, Nimrod assessed the situation. It realized via sensors its location and pace in time – which was not where it was, nor where it was supposed to be. Further, from a sign promoting the Dazzler movie, which had been sprayed with graffiti of “Mutie die!” Nimrod computed that if mutants exist, then true to its programming it must seek them out and obliterate them.
Despite this directive, Nimrod’s first action was to transform itself into as close an approximation of human as it could, most likely as not to unnerve the recovering Jaime Rodriguez. Though still alive, Jaime was disheveled from the attack and invited his apparently human rescuer, “Nimrod,” back home – possibly as Jaime would have had difficulty returning on his own.
The exact interaction between Jaime and Nimrod is not clear, but it is apparent that Nimrod maintained his façade of humanity and explained to Jaime that he was basically homeless. Grateful for his life and a good-natured man, Jaime invited Nimrod back to his house to stay. There, Nimrod ingratiated himself to the Rodriguez family, helping Jaime’s son, Tomas, with his studies and impressing the family with his ability to use the family computer. [Uncanny X-Men #197]
However, when the family was not watching, Nimrod’s use of the computer – hopelessly primitive by Nimrod’s standards – was to reach out to higher-order mainframes via the modem, allowing it to learn more about the time in which it had arrived. Quickly, Nimrod realized that there existed significant anomalies between known historical fact and the current time’s own. Nimrod concluded from this that it had moved cross-time as well as into the past, meaning that it was in an alternate reality from its own.
This new information elicited a quandary from the unit. As its primary programming was to defend humanity – which in its own time required the extermination of mutantkind – did the same hold true in this different timeline? Though Nimrod had the ability to ask the question, it felt it lacked the data yet to make a decision. However, when it then spied a news report that the super-villain known as the Juggernaut had been sighted in Lower Manhattan, the Nimrod unit was easily able to determine that its duty was clear in that criminals such as this super-villain, as well as the uncanny X-Men, were to be terminated. [Uncanny X-Men #193]
Part Three: To Serve Man
Nimrod’s first opportunity presented itself in short order. However, ironically, it was at a moment when the Juggernaut was not engaging in 
criminal activity. After locating his quarry in a bank, where the Juggernaut was actually attempting to transact legitimate business, Nimrod teleported itself inside, announcing to the human patrons therein that criminals were at large, whose duty it was to deal with. In addition to the Juggernaut, Nimrod detected two of the X-Men, Kitty Pryde and Rachel Summers, who had actually been covertly surveiling the Juggernaut since his entrance. By their very biological nature, Nimrod considered them criminals, with the sentence being death.
Due to his invulnerable nature, the Juggernaut shrugged off Nimrod’s initial assault, which merely blew away the villain’s civilian disguise. However, when the Sentinel used another blast to propel the Juggernaut away from civilians to an isolated construction site, the Juggernaut began to worry. Each counter-attack by the Juggernaut that followed was dealt with effortlessly by Nimrod, which even knocked off the Juggernaut’s helmet, a feat Black Tom Cassidy had promised would be unbreakable. With the helmet off, the Juggernaut became vulnerable to tight-beam, ultra-high frequency sonics.
During the fight with the Juggernaut, Nimrod had simultaneously countered the attacking Colossus, Nightcrawler, Wolverine and Kitty Pryde. However, proving her worth as a resource of acumen as well as abilities, Kitty developed a plan which she passed along to Rogue. Acting quickly, Rogue absorbed the powers of the unconscious Colossus and Nightcrawler, absorbing their powers simultaneously. Then, using Nightcrawler’s power of teleportation, Rogue teleported Nimrod’s arm, severing it and stunning the Sentinel’s systems long enough for her to deliver a powerful blow which shattered its entire body. Unfortunately for Rogue, Nimrod managed to reintegrate itself long enough to activate a teleportation circuit, escaping certain destruction. [Uncanny X-Men #193]
Returning to a place of safety, Nimrod repaired itself and plotted its next move. Rather than continuing its directive of mutant eradication or even law enforcement, it seems that Nimrod instead opted to continue its integration into human society, and specifically the life of the Rodriguez household. Though his appearance had been near-human before, Nimrod completed his transformation, now being able to pass for a Latino, most likely chosen for the same ethnicity of Jaime. Now able to pass for human, Nimrod accepted a job at a fish wholesaler with Jaime.
After his first week of employment, Nimrod joined Jaime at a diner in Lower Manhattan. There, suddenly 
two of the diner’s patrons decided to hold up the restaurant. Alone among the patrons, Nimrod rose to his feet to challenge the miscreants and, much to their surprise, transformed into his true form of a Sentinel before their eyes. Before they could react, Nimrod passed sentence and executed the two, reducing them to ash.
Jaime reeled at not only the true nature of his friend, but also his actions. However, before he could pass judgment, the rest of the diner’s patrons did so, unanimously thanking him and declaring him a hero. For a man of principle such as Jaime, it might have been the only thing that continued their friendship. [Uncanny X-Men #197]
Over the next few months, Nimrod remained idle in its activities as a Sentinel. Then, one night while scanning the isle of Manhattan, it detected its primary quarry, that of Rachel Summers. As a Class Omega mutant, as labeled by Nimrod’s programming, Rachel Summers would be on a par with the capabilities of Nimrod’s. However, on this occasion, Nimrod detected Rachel to be alone in the isolated Central Park and “at far below optimum performance levels.” Before departing for the hunt, though, Nimrod was called by Jaime’s son, Tomas, who asked if he could check his homework. Citing business downtown, Nimrod declined but promised to look it over before school in the morning. With Tomas returning to bed, Nimrod transformed and took off. However, lost in thought – albeit from a brain of artificial origin – Nimrod became puzzled as to its enjoyment at helping to teach Tomas, as well as the very fact that it felt any emotion. It was at this time that Nimrod for the first time called itself “I” instead of “this unit.” Clearly, Nimrod was no longer the same Sentinel unit prematurely activated not so long ago.
By the time Nimrod arrived in Central Park, the situation involving Rachel Summers had changed. Beforehand, Nimrod had detected Rachel’s status of being below “optimal performance levels.” In fact, she had been near death. Acting on her own, Rachel had attempted to assassinate Selene, Black Queen of the Hellfire Club, for her murderous actions. However, rather than see her go down the path preceded by Jean Grey as the Phoenix, Wolverine dealt a fatal blow to Rachel. Now, only through her telekinetic abilities was Rachel able to keep her heart whole and pumping and her lungs filling with air. [Uncanny X-Men #207-208]
As Rachel made her way through Central Park, two opposing sides converged on her. One was the Inner Circle of the Hellfire Club, who hunted the one who had attempted to murder one of their number. The other was the X-Men, who wished to save Rachel if Wolverine’s actions had not yet killed her. The two groups came upon Rachel almost simultaneously, with the situation quickly degenerating into a brawl between the two rival factions. However, the two group’s hostilities immediately ceased upon arrival of Nimrod, who announced that their physical nature automatically branded them felons, guilty of a capital crime punishable by death. It was during this confusion that Rachel disappeared into the night, only to be found and kidnapped by Spiral.
Back in the park, the X-Men and Hellfire Club decided to join forces. The sight of Nimrod having disintegrated Friedrich von Roehm, the Black Rook, had been enough to cause both sides to set aside all difference. Despite their combined forces, however, Nimrod seemed unstoppable. All attacks seemed to be effortlessly countered. However, in the end Nimrod was defeated in part by a combination of multiple attacks – and luck. Under orders, Harry Leland had increased the mass of the Sentinel, bringing Nimrod close to the ground, right into the hands of Colossus. While Colossus grabbed and shattered Nimrod’s leg, Shadowcat used the electrical disruption properties of her phasing powers to wreak havoc with the Sentinel’s systems.
As damaging as these attacks were, Leland dealt a nearly final blow with his own final act. Using the last of his strength, which had begun to wane from suffering a heart attack during the fight, Leland increased the mass of Sebastian Shaw, whom Nimrod had propelled to the sky earlier. Now falling like a meteor, Shaw impacted with the immobile Nimrod, who was held fast by the power of Selene. Now seemingly reduced to scrap, Nimrod faced the coup de grace of Wolverine’s adamantium claws. However, before the fatal blow could be delivered, Nimrod again teleported away to safety. [Uncanny X-Men #209]