Depends on what mutants tbh. There's a lot of weak ass mutants and then we have mutants like Legion, Magneto, etc... if we are including those showings, then it'll take someone at or above skyfather level to repeat what Uranus did, and no, not Darkseid.
My initial reaction was to deride this but upon further thought... I mean, you're right, why would Flash have the Muramasa blade in this scenario?
Even if you replaced it with a DC analogue like the God Killer sword... the thread asks which DC character could do what Uranos did. It does not ask what DC character w/ random weapon could do what Uranos did.
Having said that, it's entirely fair to recognize that Uranos had full access to his doomsday Eternal weaponry when he did what he did.
^ Pretty sure the OP was referring to the same mutants that Uranos killed during A.X.E.: Judgment Day #4-5. Uranos kinda killed everyone on Arakko. Not sure if there were a million mutants but there was an entire population of battle-hardened mutants there. And mutants like Cable, Storm, Magneto and Legion tried to fight Uranos.
^ The X-Men writers, Hickman, Ewing and Gillen, definitely spent a lot of time portraying Arakko. Could probably list a dozen+ Arakko mutants and their collective feats as a population if I closely re-read their entire run.
Uranos was eventually defeated only by the combined Omega circuit of Magneto/Storm (which killed Magneto) and Stark hacking Uranos' own weapons upon him. But I think the biggest hurdle is Legion. Like, everybody else is sorta irrelevant.
It’s always interesting thinking of stuff like this, because villains are typically derided for always losing and yet villains are who get feats like this, because a hero obviously would never.
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I like how Flash getting the blade is the issue, not whether he can actually do it once he does so.
But it shows just how trivial a million is to someone with superspeed. Ok, Legion is a tough nut - replace him with one of those generic other mutants, then. Ok, the Muramasa blade is a challenge to get - replace it with another haxx weapon (golden lasso?? Whatever) or a simple sword. Or even Speed force energy, or overloading with Speed force energy, or BFR into the speed force until they die, or phase them out of time etc.
Point is, it's the Flash, and a million mutants could die in a literal second, let alone a thousandth or a millionth of a second. I just wanted to point out the insane scale of time magnitudes at play.
Flash, even without a HAXX weapon, could easily kill a million mutants (as long as they didn't have the ability to manipulate space/time or have superspeed at his level themselves).
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and that's with a speedster who doesn't have the feats Flash has.
Even then, you'd need a mutant who is always 'on' and at that level of speed/controlling time/space. If they're just chilling somewhere, going about their normal day at normal human speed, then they're chucked even then.
Apologies, I want to make sure I get the details correctly here. So Magneto, Storm, and Iron Man teamed up to beat Uranos? Was this during his fight with other mutants, or afterward?
BTW, I think Flash is a good starting point for the discussion in question.
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Stark hacked his portals, but needed an EM charge 'like you wouldn't believe'. Storm and and heartless (literally) Magneto created a massive charge to force the portals open, enabling Stark to hack them and give control to Mags.
Who then opened portals from a dead star to fire on Uranos. Basically a variation of Darksaint85's Sunbuster tactic from using the Spot.