The First Engine
In an alternate future, Galactus was kidnapped by The Black Celestial after Doctor Strange's Images of Ikonn spell caused the devourer of worlds to have a massive, lethal stroke.
With the Deviants aiding him, the vile and treacherous Celestial fused Galactus and his matter conversion technology with an artificial black hole and increased Galactus' hunger as well as his size astronomically using his own powers.
Galactus began to consume all matter in the universe while the Black Celestial gathered enough power to set himself free from his imprisonment beneath the Diablo Mountains.
The Shi'ar Armada arrived at the site of the engine, hoping that their combined military might would be able to destroy the engine and stop Galactus. But before Empress Lilandra and her soldiers could do anything, the Black Celestial arrived on the scene and teleported them into the black hole.
While Galactus continued to carry out his captor's commands, a group of heroes (Iron Man, Thor and The Fantastic Four) from Earth-616 along with that timeline's Gladiator were already beginning to unravel the monstrous Celestial's plot.
After gaining a great deal of data vital to the Black Celestial's plans as well as a major history lesson that explained his reasons for carrying out this insane crusade to annihilate the universe, the heroes of 616 confronted the Black Celestial and threatened to travel backwards in time to warn
Arishem the Judge and the other Celestials of the catastrophe that would occur in their future. Infuriated, the Black Celestial chased after the 616 heroes across all of history while Gladiator sacrificed his life to destroy the facility that was slowly restoring the Celestial's power back to its former levels.
Because he was filled with so much rage, hatred and desperation the Black Celestial did not realize what had happened and that he was quickly draining all of his remaining power. When the 616 heroes returned to the present, they dove straight towards the black hole and quickly teleported themselves out of the way thanks to the power of Mjolnir. Due to his major loss of power, the Black Celestial was unable to do the same and plunged head first into the black hole.
After absorbing the Black Celestial's life essence into himself, Galactus momentarily regained control of his own mind but was unable to free himself of the Celestial's contraption. With no other options available,Mr. Fantastic handed the Ultimate Nullifier, which he had taken from the devourer's worldship sometime prior to the Black Celestial's defeat over to Galactus, who then proceeded to destroy himself and all of existence.
The Second Engine
Five Hundred Years after the end of The Marvel Age, Earth began to die and without the technology required to build any starships that could ferry the eight billion inhabitants off-world Humanity's survival looked bleak.
However, Robert Bruce Banner Jr., the adoptive son of Wolverine, immediately devised a plan to create a time machine that would allow them to transport the entire human population back in time to Earth-616. Many were skeptical of his plans success rate but in the end they agreed to help execute it for the sake of the eight billion lives they had sworn to protect.
Robert's plan was to lure Galactus, the Devourer of Worlds, into a trap he could not resist by putting out enough energy to make their dying Earth seem a very appealing source of food for the cosmic glutton.
When Galactus arrived to consume the world that had eluded him for so long, he came under attack from Earth's heroes, and responded in kind by systematically killing off as many heroes as he could. Though he managed to kill nearly all of the super-humans, seven heroes survived and were finally able to bring Galactus down after a long, arduous final battle.
After lobotomizing him, Robert used all of their technological resources to tap into Galactus' power and send both him and his group, The Nu-Defenders, into the past where they would have access to the technology required to create a time machine powerful enough to transport eight billion people across the time-stream.
After having gained the technology required to build what would soon become known as "The Galactus Engine", The Nu-Defenders set out on an endeavor to capture Doctor Doom and The Human Torch, which they did and then turned them into "batteries/power conduits" for the Engine by fusing them with the ex-Devourer.
The Engine was soon activated and delivered untold pain to all three living parts of The Galactus Engine, though only Doom managed to keep calm and refrain from screaming in unison with the agony-ridden Galactus and Johnny Storm, a feat which greatly impressed The Nu-Defenders.
When the rest of The Fantastic Four arrived to save Doom and The Human Torch, they found that The Galactus Engine could not be turned off nor could they free the two captives whose lives were in danger the most without causing the Engine to explode and tear the Universe asunder. Nevertheless, they could not allow eight billion inhabitants of another Earth to be transported to the already near-overpopulated Earth-616 and so, Mr. Fantastic rerouted the coordinates they had been sent and teleported the refugees to Nu-Earth.
Nu-Earth was an artificially constructed and currently uninhabited exact duplicate of Earth-616 that had been built on the other side of a parallel universe, so very far away from the native Earth of that Universe.
The reason for its construction was because the richest people in the world had discovered that a catastrophic event in the year 2012, the same one that had been predicted by The Mayans, was going to occur and bring untold death and destruction to Earth and its people, whom the rich and corrupt would leave to die so that they could breed a "purer" race of humans.
Though Doom and The Human Torch managed to survive their ordeal, the lobotomized Galactus did not and had become nothing more than a giant super-dehydrated corpse donned in scorched, partially tattered purple armor. After the incident, Galactus' corpse was secretly buried in an underground cavern deep below Earth's surface by Mr. Fantastic to mark the edge of Earth's underworld.
The rest of The Fantastic Four and The Mole Man learned this when they ventured through the same cavern to reach an old, failed experiment that The High Evolutionary had forgotten to shut off when both he and his followers fled the city that had been built atop it. The corpse was later discovered by The Silver Surfer who reported its existence and the existence of The Fantastic Four's energy signatures emanating from it to his Master. Furious and fraught with questions that demanded answers, Galactus and The Silver Surfer confronted Mister Fantastic atop The Baxter Building about the corpse's presence on Earth-616.
The Third Engine
The Third Galactus Engine was once Galactus' counterpart from The Cancerverse until The Many-Angled Ones of the outer dark took him apart, amplified the power within his corpse and bio-engineered an apocalyptic mechanism, an engine of cosmic annihilation unlike anything the Multiverse had seen before.
When The Many-Angled Ones, from behind the scenes, learned that Earth-616's Galactus had brought Tenebrous, Aegis and The Prime Celestial Host with him to The Fault to keep them from achieving their dreams of conquering Earth-616, they sent The Galactus Engine out to deal with the situation.
When it made its grand entrance, Nova, Quasar and The Silver Surfer were shocked to discover that the foul abomination dwarfed every single cosmic being stationed at The Fault in size, making them look like ants much in the same way Nova, Quasar and The Silver Surfer were to the cosmic beings.
When The Galactus Engine finally broke through The Fault; all of the cosmic beings standing before it moved aside in order to avoid being hit with the exception of Galactus himself, who decided to face his twisted counterpart head-on.
Shortly upon arrival, The Galactus Engine did not seem to be attacking anything at all but in reality was waging a fierce war with the Abstracts on a higher plane of existence far beyond the ken of mortal men.
However, when it overwhelmed Aegis and managed to kill her, the end result was viewed on our plane of existence in the form of a massive xenomorphic entity tearing its way out of her body. It was not long before her allies as well as the mortal armies of 616 began to drop like flies.
Though it was not seen firsthand, Quasar remarked that The Galactus Engine had actually begun forcing the cosmic entities and their mortal allies into retreating, though because of all the confusion and chaos going on at the front lines it was impossible to tell just how many of the cosmic beings had actually fled from the front lines.
Following the return of Nova's strike-force from their mission to rescue Major Victory and Namorita, Queen Medusa explained to them that The Galactus Engine had proved to be the most powerful and monstrous threat on the battlefield so far, which had wiped out or forced into retreating all but five of the High Abstracts that had come to assist the lesser beings in fending off the Cancerverse's assault vanguard earlier on.
The five remaining High Abstracts; Galactus, Arishem the Judge and three nameless Celestials who, despite focusing every iota of their vast power into combating The Galactus Engine, were noted in the reports Medusa received from the front lines as being barely able to hold The Galactus Engine back.
To make things even worse, other reports had confirmed that there were larger, more powerful creatures (possibly The Many-Angled Ones themselves) coming through The Fault from behind The Galactus Engine and the two parsec-wide assault vanguard flanking it.
Eventually, Arishem and the other three Celestials were overwhelmed and possibly destroyed by The Galactus Engine, leaving only Galactus to fight back against his monstrous counterpart. But as their fight progressed, The Galactus Engine unleashed hundreds of robotic tendrils from its mouth and began wrapping them around Galactus' arms.
As it began to pull him downward, Galactus found himself being assisted by the lesser beings fighting in this war, who had all decided that The Galactus Engine should be everyone's priority now that the other High Abstracts had been killed. But even with the promise of what little aid they could provide, the outcome of this fight still looked grim.
Then, when all hope seemed lost, Mistress Death obliterated The Cancerverse and caused The Fault to collapse in on itself, creating a back-rip that pulled The Galactus Engine and any portions of the assault vanguard that hadn't turned to dust down into oblivion. With Mistress Death's aid Earth-616 had come out of the war victorious, but the cost of that victory had been great.
The Galactus Engine had left every species that confronted it, including The Celestials, in ruins. The Celestials themselves now lacked their primary Judge thus leaving Arishem's position along with countless others to now be filled, at least until they could recover from the war.
As for the "lesser" races...millions upon millions of people, people that were loved and cared for deeply by many, had been exterminated in the blink of an eye by forces they could not understand or imagine.
Though it is gone, The Galactus Engine's dark and twisted legacy along with records of its actions will be remembered by every generation until the end of time and space and light and life.