Of the Kang the Conqueror. Each desires complete dominion over the Earth, which means that when Earth's Mightiest Heroes can no longer stand in their way, they're destined to fight it out in an event known as the Ultron War, ultimately destroying all of time.
The Ultron War was revealed in Avengers vol. 4's 'Next Avengers,' from Brian Michael Bendis and John Romita Jr. In this ambitious story, the Avengers are suddenly overwhelmed by a wave of time-displaced beings - some from the past, some from Marvel's many possible futures. Splitting into teams, Bucky Barnes' Captain America, Iron Man, Protector, and Wolverine travel into an apocalyptic future, where an older Tony Stark explains the unavoidable threat created when Ultron and Kang go to war.
The older Tony explains that Kang, obsessed with conquest over a worthy foe, eventually realizes that Ultron is the ultimate opponent. Constantly improving his intellect and physical form, Ultron will one day become more powerful than any mortal being, and it's at this point in time that Kang strikes, having formed an army of heroes and villains from throughout time. But having achieved a digital consciousness that can be transmitted between multiple Vibranium-Adamantium alloy bodies, Ultron proves unbeatable, prompting Kang to start manipulating time to create an army he can lead to victory.
The consequences don't happen all at once. Honorable to a fault, Kang refuses to face Ultron at anything but his strongest moment, but no matter the army he assembles, he loses, prompting more tampering with time. Through his constant manipulations, Kang eventually breaks the timestream, causing the day of the battle to start looping like an apocalyptic Groundhog Day. With time collapsing in around the Ultron War, the old Tony Stark has summoned the Avengers because, once the day starts again, they'll have a chance to face Ultron and beg the unstoppable android to let Kang win, as it's the only way to stop the looping Conqueror from destroying time completely.
While the future Ultron is initially hostile to the Avengers, he's ultimately a super-intelligent being who can't deny the logic put before him, and when the Ultron War begins again, he allows Kang to destroy him, frustrating the time-hopping villain, who goes off in search of another challenge. It's a fascinating example of how powerful each villain is, with each so supremely undefeatable that when they go to war, time itself is forced to buckle and break around them.
One of the most dangerous things about Kang is that he is limited only by his own arrogance and sense of honor. His mastery of time travel means he could kill any of his enemies in their cribs, but instead he chooses to fight them at their strongest. For the Avengers, that has meant many brutal battles they've barely survived (and even some they lost), but against Ultron, Kang finally met his match. The Ultron War was just one possible instance of a final battle between Kang and Ultron, and it cost countless lives and nearly destroyed time itself. There's potential for an identical war to begin should Kang and Ultron once more find themselves at odds, and the story may even be adapted in the MCU, where the multiverse would offer an even more ambitious ultimate showdown between the two villains. In the meantime, all the Avengers can do is try to stop Kang the Conqueror and Ultron wiping them out, since once Earth's Mightiest Heroes fall, time itself is likely to pay the price.
