Name This Marvel Villain??
If you were to suggest to this being that he classifies as a villain, he may decide to either arrest you--or incinerate you, depending on his judgment of your infraction. So try to choose your words carefully, because Nimrod is indeed judge, jury, and executioner, all wrapped up in a sophisticated package of technological might that's equipped to deal with any enemy. A Sentinel project from a future where mutants have been outlawed and all but eradicated, Nimrod is the ultimate in Sentinel technology, with abilities that give it the capability to think like and even appear as a human, allowing it to act as a more effective--and more covert--agent of law enforcement. Needless to say that, to a mutant, Nimrod is a clear and deadly threat, whose mandate is to capture or kill its mutant target(s), without fail.
Nimrod is on the verge of being brought online when Kate Pryde--the adult version of the X-Man known as Shadowcat, whom you may remember from the story where Rachel Summers assisted in sending her consciousness to the 20th century in order to prevent the assassination of Senator Robert Kelly--conducts a mission to infiltrate and destroy the complex where the unit is housed. Accompanying her is Rachel, who hasn't been told the real reason for being brought along on the mission--a convoluted story involving the Phoenix force convincing Kate to let it infuse itself in Rachel, an arrangement which also allows Kate to send Rachel to the past and give her a life much more promising than the one she currently has.
Before things at the complex heat up, Kate throws a psychic "switch" (courtesy of Phoenix) that initiates Rachel's transport back through time--while almost immediately afterward, Nimrod appears, but too late to stop Kate from detonating a nuclear device that destroys the complex. Yet before Nimrod meets the same fate, he attempts to mechanically reproduce the process that allowed Rachel's escape (presumably to pursue her), and finds himself arriving in a past that has differences from the world he's come from (that past being our past).*
(*That's the condensed version--we'd be here all day if we were to go into all the details of Kate, the Phoenix, Nimrod's detour on his way to the past, and a few other things we don't really need to sift through.)
In the past, Nimrod materializes just in time to stop a human male named Jaime Rodriguez from being knifed and robbed. (And by now you probably have a fair idea of how Nimrod "stops" a perp.) After making a quick assessment of his surroundings, Nimrod realizes that his mandate to wipe out mutants is going to get quite the workout in this time period.
Rodriguez is understandably grateful to Nimrod, to the point of taking him in while he gets his bearings. And for that, even a 1985 computer proves useful.
As we can see, Nimrod is a higher order of Sentinel model, built to operate autonomously without a "Master Mold" or other Sentinel units to coordinate with. He also takes advantage of his ability to assume a more human appearance in order to "fit in" with his surroundings, the better to conduct his operations. Rodriguez and his son, Tomas, take to Nimrod quite well, with Nimrod growing more accustomed to their hospitality and friendship and unexpectedly finding himself benefiting from this close exposure to humans. Yet his purpose remains a deadly one--and he has only to tune in the news to realize that his programming is still binding no matter what time he operates in. And there is much for him to do here.
Nimrod certainly starts at the top in his seek-out-and-destroy campaign against outlaws, taking on the Juggernaut and pivoting to the X-Men (who also arrive on the scene to end the Juggernaut's threat). While keeping Juggernaut occupied, Nimrod handily deals with Wolverine and Shadowcat, as well as downing both Colossus and Nightcrawler with one shot. And when Juggernaut renews his attack, Nimrod proves that even one of the most powerful, unstoppable bruisers in the world is vulnerable to a being that has been built to neutralize whatever foe it targets--a "super"-Sentinel who excels in tactics as well as deducing its target's weaknesses.
As for the X-Men, they're down to a man. Only Rogue remains on her feet, and she maximizes her power by becoming an amalgam of Shadowcat, Nightcrawler, and Colossus, adding their powers to her own. Taking Nimrod by surprise, she nearly manages to destroy him.
Nimrod escapes to fight another day, which arrives when the Lords Cardinal of the Hellfire Club are in search of Rachel, who had earlier invaded the Club with the intent of slaying their new Black Queen, Selene--that is, until Wolverine mortally wounded her. Subsequently, the Lords run into the X-Men in Central Park, and a battle seems imminent--though the battle will be with another, who has marked all mutants in the vicinity for death.
Since its last encounter with the X-Men, Nimrod's activities against the city's many "outlaws" have made him something of a hero to the populace, in light of the current anti-mutant sentiments that have come to be embraced by many. Nimrod, of course, doesn't go through red tape in his dealings with criminals, human or mutant, its ability to gauge right or wrong and to act accordingly already being embedded in its programming; and in tandem with his successes, we've seen him exhibit an almost human pride in being of service to law-abiding humans. But for the X-Men as well as the Lords Cardinal, self-preservation trumps pride--and both join forces to defeat this deadly enemy, with one of the Lords meeting his end in order to ensure that Nimrod meets his.
Nimrod again escapes, this time before Wolverine is able to slash it to shreds. From here, he takes a three-year hiatus from Uncanny X-Men, during which time it becomes clear that he's continued to deliver his own brand of justice to lawbreakers.
Nimrod has obviously become a fixture in law enforcement, though nonaligned with formal enforcement officers (perhaps due to its routine method of dealing with criminals with extreme prejudice); yet it's curious that Nimrod hasn't pursued its hunt for mutants more aggressively, since it can track them well enough (particularly the members of the Hellfire Club, who are easily reached through their well-known address). We're left to assume that Nimrod has become comfortable with exploring a more human method of coexistence with his role as a Sentinel--keeping a low profile until he's needed, and perhaps realizing that he must work outside of the law, given that his methods and decisions are in conflict with the restrictions of this time period.
Unfortunately, he's about to be taken off the grid for an indefinite period of time, when an accidental encounter with somewhat ancestral technology merges him with another whose goals subvert his own.
As we can see, the Master Mold has reinitiated the adaptive programming that the Sentinels enabled to work around their directive to never harm humans, a directive that they reasoned was preventing them from ultimately resolving the mutant problem. The seed was planted long ago, thanks to Cyclops:
Which was later expanded on in an Avengers story:
But what the Master Mold hadn't counted on was Nimrod, whose enhanced abilities make the Master Mold an almost unbeatable foe for the X-Men, but whose affinity for humanity now works against the construct that has absorbed it. And when the X-Men decide to force the Master Mold through the Siege Perilous--a dimensional doorway that leads to judgment and rebirth in another life--Nimrod delivers the coup de grâce, in a barrage of runaway logic that any aficionado of Captain Kirk would recognize in a heartbeat.
Which would seem to be the end of Nimrod--though what goes around truly comes around, at least concerning the events of the so-called "Days of Future Past." For it's in the battle's aftermath that we learn the origin of Project: Nimrod, when a casualty of the battle against the Master Mold is revealed to be the wife of none other than Senator Kelly--for who this proves to be the last straw as far as his feelings toward the threat of mutantkind are concerned.
Senator Robert Kelly, folks--who sets in motion his own wife's death.
These scenes comprise the bulk of Nimrod's story, a killer Sentinel that never did quite fulfill its potential of being the ultimate threat against present-day mutants and whose build-up virtually fizzled out with each succeeding appearance. To connect the dots with Nimrod's other appearances would probably be pointless, except for the completist; instead, we could simply trust that the judgment he received through the Siege Perilous was just, something a stickler for the law such as Nimrod would perhaps appreciate.