Magneto is an Omega-level mutant whose powers are as memorable as his iconic cape-and-helm look. Originally an enemy of the X-Men and a global menace to humanity, Erik Lehnsherr would ultimately become an ally of his former enemies, choosing to put aside his goals of advancing homo superior to fight for the greater good. Throughout the years, Magneto has had many impressive displays of his ability to manipulate magnetic fields, but few compare to his power levels ascending to the point where he attempted to prevent two separate worlds from colliding and destroying the Marvel universe completely.
In Magneto #18-21, the "Last Days of Magneto" storyline by Cullen Bunn with art by Paul Davidson and Gabriel Hernandez Walta, Magneto and his daughter witness the Incursion between his Earth-616 and Earth 1610, otherwise known as the Ultimate Universe. Once on good terms with the X-Men, Magneto had been spending his spare time protecting and handling mutant affairs on his own terms until he was approached by Namor. Revealing his membership of the Illuminati and their efforts to understand, prevent, or select worlds to be destroyed by Incursion events, Namor asks Magneto to be his failsafe in case the Illuminati fails and an Incursion arrives. Considering Erik's powers and their shared dedication to protecting mutants regardless of the cost or methods, Namor trusts only Magneto with this task. The former villain promises to prepare by increasing his power levels just in case the smug King of Atlantis and his clandestine compatriots are unsuccessful.
In the story, Magneto and his right-hand woman Briar investigate methods for Magneto to increase his already impressive power to unthinkable levels. The first conversation is with a chemist who presents him with a cocktail of dangerous chemicals including Mutant Growth Hormone, which begins Magneto's obsessive search for improvement despite the danger it'll present to his body and mind. Presented with options from several reputable Marvel geniuses and scientists, Erik chooses the X-Men villain known as the Sugar Man. The sinister Sugar Man provides him with power amplifiers but still pays for his crimes against mutantkind with his life. With the combined MGH and Sugar Man's power amplifiers, Magneto's powers continue to grow as the world continues to crumble around him and his daughter Polaris. Tapping into both Earths' magnetic fields, Magneto grows powerful enough to not only defeat the Nimrod Sentinels from the Ultimate Universe but also steal Polaris' powers for his own, believing that it is his responsibility to prevent this cataclysmic event, even if it kills him.
An interesting point is that as Magneto attempts to use his enhanced powers to stop the Incursion, he wonders if his counterpart in the Ultimate Marvel Universe was attempting the same thing. The Ultimate version of Magneto was a fanatical terrorist who led the Brotherhood of Evil Mutants, seeing himself as a mutant Messiah willing to kill millions in the name of mutant supremacy. Magneto's efforts were opposed by Xavier's X-Men and other heroes, including the Ultimates, the latter consisting of his children Wanda and Pietro. The Ultimate Magneto accomplished a level of infamy that dwarfed his mainstream counterpart in Ultimatum when he used his powers, enhanced by the stolen Mjolnir, to create worldwide devastation, killing millions in revenge for the deaths of his children. This Magneto had been dead long before his world was set to collide with the mainstream Marvel universe, his mainstream counterpart clearly the better man for being able to put aside his prejudices in order to save everyone regardless of their genetic makeup.
Despite his unimaginable power levels, Magneto is unable to prevent the Incursion from occurring, thus killing him in the process. As Earth-616 begins to disintegrate and fade into the light, his daughter Polaris looks back at her adopted father's sacrifice and despite the fact that he failed, she still loves him for the fact that he tried. Unfortunately, the events of Secret Wars rebooted Earth-616 with little to no memory of Magneto's sacrifice - a shame, as it was truly the mutant martyr's finest hour.