Grom was born on the ancient planet Eternus, a member of a highly advanced, belligerent race of humanoid aliens who are now extinct.
Eons ago, the Eternians learned to control aging and eliminate natural death. To prevent overpopulation, the Eternian government sanctioned interstellar warfare and conquest which regularly claimed the lives of multitudes. When enemies could not be found to wage war against, the government held gladiatorial spectacles called The Games. Grom was a naturally skilled gladiator who earned in combat title Champion of Champions. Soon after Grom’s coronation, the Eternians discovered Gigantus, a huge planet on which dwelled an equally advanced but peaceful civilization. Immediately they launched an attack upon the world. Because Gigantus was several hundred times larger than Eternus (about the size of Jupiter), the war became very costly and difficult for the Eternians to sustain. Finally it was decided to simply obliterate the planet. The Eternians showered Gigantus with immensely powerful nuclear warheads and detonated them, destroying the planet.
A huge armada of Gigantians, however, who had learned the ways of war after eons of peace, escaped the world’s destruction and attacked Eternus in retaliation. Unprepared for so devastating an attack, the Eternian defenses fell. Government leaders realized that their race faced total extinction. Desperately Eternian scientists prepared to transform Grom, the race’s most outstanding physical specimen, into the receptacle of the collective consciousness and mental might of Eternus. As the surviving population of Eternus marched into huge “synthesizing chambers” scattered across the planet, their organic brains were converted to pure mental energy and projected into Grom. When the process was complete, and only the scientists who conducted the process were left alive, the capsule containing Grom was launched into space to incubate until the being learned to control the powers of his group mind. Eons passed. Then Grom emerged as the Overmind, a warrior who possessed the collective psionic power of the billion brains and who carried in his memory the subliminal command to subdue the universe.
The Overmind attacked the first inhabited planet he came upon, the Earth, but was defeated by the Fantastic Four with the aid of the Stranger. The Stranger claimed to be the embodiment of the collective psionic power of all the surviving Gigantians. The Stranger’s psionic abilities proved to be more powerful than those of the Overmind and he shrunk the Overmind to microscopic stature and exiled him to a mote of dust. Several years later, a demonic creature called Null the Living Darkness located Overmind and decided to use him as a pawn in his scheme of conquest. However, when he restored the Eternian to his original power and stature, Null accidentally transported the Overmind to an alternative Earth other than the one upon which he had met his defeat. This Earth was the home of the Squadron Supreme. Null and the Overmind battled the Squadron, who sent for reinforcements in the persons of the Defenders. A being composed of the psychic union of six of the alternate Earth’s most powerful telepaths also aided in the struggle, and were instrumental in neutralizing Null and the Overmind. At the battle’s end, the Overmind’s Eternian consciousness was suppressed and the six telepaths, lacking physical bodies, took refuge inside him. Now possessing a benevolent mind and a small fraction of his former mental might, the Overmind returned with the Defenders to their world, joining their ranks for some months. After parting company with the group, he set forth for parts unknown and eventually spent some time trying to help a town full of people suffering from radiation poisoning by granting them an illusion of health.
Years later he resurfaced, Grom's consciousness having returned and overwhelmed the telepaths, and took control of the Squadron Supreme again (the Squadron now being resident on Earth-616). He used the Squadron to attack the Stranger's laboratory world but was defeated and captured by the Stranger. He eventually escaped and returned to Earth. For a time, the Overmind was under the mental domination of the Purple Man, who used him to monitor the population of New York. More recently, he was briefly forced to work with Baron Zemo.
The Overmind of Earth-712 resided for centuries, aware of his counterpart's attempt to take over his universe, until he one day befriended the Squadron's arch-enemy the Scarlet Centurion. When he was brought back to the 20th century to combat the Nth Man, he ultimately perished from the mental backlash caused by his attack on the cosmic threat.