The Marvel Universe's heroes have faced off against countless deadly foes but few have brought the entire planet to its knees the same way as Ultron. Ultron is a robot of immense power and intelligence who quite often uses this intellect to create a mechanical army to take on his usual foils, The Avengers, but what if he didn't have to create the army? Marvel has missed a major opportunity for Ultron by never taking over The Sentinels, robots created to combat the mutant threat and usually involved with the X-Men.

Ultron was created by Hank Pym and debuted in Avengers #55 (by Roy Thomas, John Buscema, George Klein, and Sam Rosen) yet has almost exclusively been an Avengers antagonist all this time. Having Ultron take over The Sentinels would be a fatal step none of the heroes would see coming and from there he'd have an army of robots tailor-made to adapt and neutralize powers. This would give him an edge over Earth's Mightiest Heroes who aren't used to fighting Sentinels whatsoever.

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As The Avengers struggle to adapt, the X-Men would go through the exact same issues due to the upgrades and innovations Ultron would employ. In stories past, he's upgraded his body to be made of adamantium, which, if employed in Sentinel production, would create an army impervious to almost all damage. It's these advancements that make every fight with Ultron deadlier than the last. Imagine a world in modern Marvel Comics where the X-Men are doing their usual business on Krakoa and out of nowhere the sky begins to darken as a swarm of Ultron-enhanced Sentinels blot out the sun. At the same time all the major cities around the globe begin to experience the same thing, a massive army moving into position to start the final purge of humanity Ultron has always thirsted for.

The Avengers, The X-Men, Fantastic Four, The Inhumans, everyone would need to be on hand for a threat of this magnitude. Marvel would have a major cross-over event unlike anything they've done so far. It'd be a catalyst to get every hero to team up against the robots and this is without even considering Nimrod's involvement. Nimrod is an advanced Sentinel prototype that is virtually unstoppable due to his ability to nullify super-powers. Given the threat of Ultron and The Sentinels, even the villains would likely be recruited to the cause to stem the robotic onslaught, adding to the scale of the cross-over.

Marvel has definitely had some incredible stories centered around this mechanical menace and this deadly step seems so obvious, yet, Ultron has had next to no direct interaction with Xaiver's team and the rest of the mutant population. The evil genius is always plotting, always working on his next run at genocide which coincides with The Sentinels main purpose for existence. One would think that at some point he would discover the blueprints for The Sentinels during his exploration of the internet and technology, especially as he explored government files.

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Ultron absorbs Odin's power

He could even use the extensive information he finds about The Sentinels and their engagements with the X-Men to develop counter measures against super-powers similar to Nimrod's. These schematics could grant him acccess to The Master Mold or Mother Mold as well, meaning his nigh unstoppable army would be able to automatically replenish itself, We've seen storylines where The Sentinels have decimated the planet with Days of Future Past and the same for Ultron with Age Of Ultron bt this would switch things up in a way fans have never seen.

It's always interesting to see heroes and villains fight one another that normally never interact, and seeing Ultron take the fight to the X-Men would change how he's typically used. The character doesn't get the due diligence he deserves for how devastatingly powerful he truly is and wiping the floor with the world's premier mutants would showcase that fact quite well. Marvel has ignored the opportunity to utilize Ultron to his fullest potential, but maybe they'll one day realize this folly and finally elevate its premier robot to be the ultimate villain.