With Avengers: Endgame fast approaching, we are finally going to see how the Avengers manage to find a way to stop Thanos and the staggering power of the Infinity Gauntlet, which was just used to kill off half of the universe. The sheer magnitude of the Infinity Gauntlet's destructive abilities has caused many fans to be fascinated with the cosmic device.
At the same time, though, the very nature of creating a weapon that is "the most powerful" has led to the obvious question of, "Okay, but is there seriously nothing else more powerful than it?" The truth of the matter is that there appears to be one other cosmic item within the Marvel Multiverse that is considered more powerful than even the Infinity Gauntlet. Its existence, however, is a matter of controversy in and of itself.
While he has done many famous comic book stories over the years, it is clear that Jim Starlin is now best known for being the writer behind the Infinity Gauntlet, the 1991 crossover where Thanos put together the aforementioned collection of the Infinity Stones (then known as the Infinity Gems) to wipe out half of the universe.
Since then, Starlin has done a number of follow-up stories that feature both Thanos and Thanos' heroic counterpart, Adam Warlock. His first two follow-ups, Infinity War and Infinity Crusade, directly followed Infinity Gauntlet in 1992 and 1993, respectively. Starlin then took a decade-long gap before returning to Marvel in 2003 with Marvel: The End.
The End was a series of titles that Marvel released between 2003 and 2010 that purported to show the "final" story featuring Marvel's various heroes. However, Starlin took it a step further with his six-issue series titled simply Marvel: The End (written and drawn by Starlin and Al Milrom). The concept of the series is that the supreme being in the Marvel Multiverse decided that there were too many superhero resurrections. It was tipping the universe out of balance, as there were more people alive than should be alive. So a powerful weapon called the Heart of the Universe was created to help balance things out.
An Egyptian pharaoh by the name of Akhenaten was given the weapon and Akhenaten proceeded to wipe out most of the heroes in the Marvel Universe. However, the remaining heroes, along with Doctor Doom (who wanted the weapon for himself) contested Akhenaten and in the end, the weapon found itself in the hands of Thanos.
This was not something that the remaining heroes (and the cosmic beings of the Marvel Universe) were cool with happening...
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The main hook of the Heart of the Universe, and the reason why it is the most powerful weapon that there is is the simple fact that it was part of the universe itself. The Infinity Gauntlet was such a powerful weapon because it had the ability to alter the universe. However, it was not literally a piece of the universe. It just affected it like any other cosmic weapon.
This is why, for instance, the cosmic being known as the Living Tribunal was able to pass laws that would affect the Infinity Gauntlet, like its determination during Infinity War that the Infinity Gems could no longer be combined to form the Infinity Gauntlet.
That particular ruling was promptly ignored by most later writers, but the basic rule of thumb applies, which is that the Living Tribunal was powerful enough to control the power of the Infinity Gauntlet.
However, since the Heart of the Universe is a piece of the universe itself, then the Living Tribunal is a part of it, as well. Therefore, the Heart of the Universe is even more powerful than the Living Tribunal.
Thanos discovered this when he wiped out the heroes who were opposing him in Marvel: The End #6...
This was after he destroyed a Celestial like it was nothing!
When the Living Tribunal determined that Thanos could not be allowed to wield such a weapon, Thanos simply wiped the Tribunal out, as well...
Eventually, Thanos discovered that the aforementioned imbalance to the universe had resulted in a flaw that was killing the entire universe. Not even the Heart of the Universe could stop it because, of course, as noted earlier, the Heart is part of the universe (so it couldn't cure itself). There was only one thing left for Thanos to do, which was to wipe out the universe entirely. Thus, it was soon just Thanos and utter blackness. Luckily, though, Adam Warlock was between universes at the time, and so Warlock was able to meet with Thanos and reason with him that it was Thanos' duty here to actually repair the universe and return it to its previous state.
The big difference is that when Thanos put everything back into place (with all of the heroes returning to the living), he would also erase all existence of the Heart of the Universe. The only two people who would remember what happened (and that the Heart existed) were Thanos and Warlock.
The trick here, though, is that Marvel: The End was very much an out of continuity exercise. Unlike the other books in the series, it showed a story set in modern times that appeared to be basically the main Marvel Universe. However, it was not treated that way by any other title. The only references to the events of The End since it happened was in Starlin's own later works involving Thanos, including the original graphic novel, Infinity Revelations.
However, since Marvel's alternate reality stories all exist within the greater Marvel Multiverse, the Heart of the Universe still counts as part of the overall Marvel Multiverse and can be fairly compared to the Infinity Gauntlet. It is simply a matter of noting that within the traditional "in continuity" Marvel Universe, the Heart of the Universe has never existed and very likely never will exist.
Still, Multiversal weapons "count" when discussing the most powerful Marvel weapons and so the Heart of the Universe wins top marks when it comes to ranking the most powerful weapons from Marvel (as we noted a few years back in such a list).