House of M is one of Marvel's most popular events of the century, mainly because of the "No more mutants" spell. This spell caused mutants to lose their powers, which also became known as M-Day.
However, there has been some misinformation about the spell and its impact/effects. The idea that the spell affected worlds and dimensions beyond Earth-616 came from Dr Strange, but has been proven to be inaccurate. Before I get into why it is inaccurate, I want to point out that Dr Strange was an unreliable source from what we saw in later events. Dr Strange assumed that an attempt to break the spell or reverse its effects would cause reality to implode. According to him, the spell was woven through reality and couldn't be reversed, but Marvel didn't seem to think that he was right. Or they didn't care and went on to constantly break the spell as though that statement didn't exist in the first place.
For example Xavier got his powers after entering the M'kraan Crystal and even got a permanent boost from this:
Magneto got his powers back via celestial tech:
Polaris got her powers back after she stopped being a Horseman of Apocalypse:
The Phoenix Force went on a streak of creating new mutants and later broke the spell on the planet. Note: The spell prevented mutants from being born, which was why mutants were almost extinct in the future timelines that were explored.
The Five were able to resurrect depowered mutants with their powers restored, such as Prodigy:
and Aero:
As we can see, the effects of the spell were reversible, which makes Strange's explanation seem unreliable. This also begs the question: is he also right about the spell affecting billions of souls?
Marvel already answered this. They didn't even bother to show the spell affecting a single alternate universe but went on to show us alternate universes that the spell did NOT affect. Now I know that some people will mention editorial stuff or that Marvel didn't want the spell to affect the stories that they wanted to tell... but they could have shown the effects in other universes to support Dr Strange's claims, and they didn't. On the contrary, they went on to reference the spell and show that it never affected other realities.
Marvel released a "What If...?" comic that was based off House of M, but instead of "No more mutants", Wanda said "No more powers", which depowered every superhuman. This is an alternate universe that 616 Wanda's spell did not affect.
When 616 X-Force visited the Age of Apocalypse reality, they told AOA Jean about the "No more mutants" spell, and this inspired her to use her telepathy to make AOA Wanda cast the spell, although it didn't work well:
This shows that the spell hadn't affected the AOA reality.
Ultimate universe mutants were born from a human experiment, and a cure was developed, reducing their population to a handful of mutants. This happened after House of M and shows the Ultimate universe was not affected by Wanda's spell.
There have been alternate realities that had mutants that weren't affected by M-Day. The Exiles, X-Treme X-Men, and two other X-related teams traveled to alternate universes, and saw that mutants there were unaffected by M-Day. If Marvel made a mistake, then why did they repeatedly show universes with powered mutants and no mention of depowered mutants, even in X-Men comics where the "No more mutants" spell was a big deal, even after AvX? If we're going to support the claim that the spell affected all realities, then at least show a reality where it happened, right?
The other statement of the spell being omniversal was from Beast, and he wasn't certain. How do we know that he wasn't certain? Beast said: "...you said three words that robbed nearly every mutant on the planet -- and perhaps even the omniverse -- of his or her mutant abilities."
"Perhaps", which denotes a lack of certainty.
The last argument that I have heard to support that Wanda's spell affected the omniverse was that she pushed the Phoenix beyond time and space; not only is that irrelevant to the spell, but false and weirdly misinterpreted.
Unit said: "It was pushed. This is the Phoenix pushing back."
When Unit was referring to the "No more mutants" spell and said that it pushed the Phoenix, it meant that the spell interfered with the universal constant that is the Phoenix. Wanda's spell was the chaos to the Phoenix's order. The "cosmic entity" in question is the Phoenix, a being beyond time and space. Unit wasn't saying that "No more mutants" spell was beyond time and space, nor that it was pushing the Phoenix beyond time and space.
Marvel has also stated that the spell was worldwide/ planetary on different occasions.
When the Phoenix dispersed its energies to recreate the mutant race, its energies were dispersed across the globe, not the multiverse, which proves again that it only affected the Earth:
Exodus said that she depowered one million mutants:
History of the Marvel Universe #5 (from 2019) described Wanda's spell as "transforming global reality" and "the near-eradication of mutant powers worldwide":
I know that this will come with a lot of backlash. If you disagree and have a scan where it shows another universe in which Wanda's spell depowered mutants, feel free to post it.
I wasn't planning on making this thread, but there's been a lot of misinformation, and those spreading this misinformation are very loud about it. These days, loud = right to many people. The evidence supporting the legitimacy of the spell being multiversal/omniversal comes from an unreliable source, an uncertain source, and a statement unrelated to the range of the spell.