One multiverse version of Doctor Doom gained the powers of Galactus and proceeded to wipe out almost all life in one Marvel Universe.
During the period following Secret Wars when the world believed Mister Fantastic, the Invisible Woman, Franklin Richards and Valeria Richards were dead, the Thing was alerted to the existence of the Multisect — a device capable of allowing people to travel the multiverse. Alongside the scientist Rachna Koul, Thing and Human Torch ventured into the multiverse, nominally in search of their family, at least according to the Thing. One of the first worlds they encountered included one of the most dangerous versions of Doctor Doom in existence, a variant of the longtime villain who had not only claimed the Power Cosmic but effectively transformed himself with the power of Galactus.
In Marvel 2-in-One #4-6 by Chip Zdarsky, Jim Cheung, Walden Wong, Frank Martin, and Joe Caramagna, the ultimate Doctor Doom/Galactus fusion was introduced, and even though it made Galactus into an even greater threat to the universe, Doom wasn’t able to even enjoy his newfound power.
In this reality of Earth-18466, the divergence occurred when Galactus first came to Earth. In this reality, the Thing charged into battle against the cosmic force far more brazenly — and was reduced to a pile of rocks by Galactus. Reed was so consumed with rage that he didn’t become the savior of Earth, which left the door open for Doctor Doom to intervene. Using new technology, Doom was able to effectively take over the body of Galactus, transforming it into a unique new state. While Galen promptly went mad within Doctor Doom’s body, the now cosmically powerful Doom vowed to never consume the Earth. Instead, he departed for the rest of the galaxy and began to consume countless worlds, leaving nothing but Earth and its Sun (in actuality the Human Torch of this reality) in the entire galaxy.
Within years, nothing else remained in the universe, as every other planet and star is quickly consumed by Doom. Utilizing a legion of cosmically empowered Doombots as his “Healrds of Doom,” this variant of Victor Von Doom grew in power and hunger and was eventually forced to return to Earth to eat it as well. By the time he arrived, he found himself facing the superhero community of this world, united under the banner of SHIELD with Invisible Woman at the head. Meanwhile, Reed Richards — left by Susan and wallowing in self-despair after the loss of his best friend — had spent years trying to help the world and find a way to deal with Doom. Thanks to the arrival of the Earth-616 Doctor Doom — then trying to serve the world as a hero — Reed was finally able to concoct a plan to fight back against Doom.
While the forces of Earth stood against Doom, a Pym Particle enhanced Thing was able to grow massive enough to actually force Doom into a physical brawl. Consuming the energy given off by his reality’s Human Torch, Doom’s hunger is briefly saited long enough for him to reveal to Reed Richards that he genuinely doesn’t want to eat the Earth — but that the hunger is too much for him to bear. This opening gives the Emma Frost of this world (and the wife of the now depowered Silver Surfer) the chance to activate Doom’s machine once more and trade bodies. While the Victor Von Doom dies within Emma’s body, Emma (and a “trojan horse’ planted into her mind) reversed the purpose of Galactus and turned the world devourer into a life-bomb — creating new worlds and stars with its power.
Emma soon departed to travel the cosmos with a re-empowered Sufer by her side, bringing new life to the universe that Doom had come close to consuming. Notably, even in his most powerful form, Doom wasn’t happy. His cosmic power did nothing to improve his mood, and his final moments spent telling the Earth-616 Doctor Doom that they will always be hated by the Fantastic Four did little to give the man peace.