The Over-Mind was once an Eternal named Grom. He lead the interplanetary conquerors known as the Eternals to victory as they enslaved a thousand worlds. He also faced the challenges all Eternals faced to amuse themselves once most worlds in the universe were conquered. He was the best of the best and dubbed the Champion of Champions. The Eternals finally faced defeat when they attacked the enormous world Gigantus. Selecting Grom to be the sole survivor of their race because of his status, the Eternals transferred their mental energies to him, transforming him into the Overmind. It took thousands of years of incubation for him to awaken.
Overmind was created by Stan Lee and John Buscema and first appeared in Fantastic Four Vol.1 issue 113 (1971).
Major Story Arcs
Meeting the Fantastic Four
He then awoke and made his way to Earth. Here he turned the city of New York against the Fantastic Four, for he believed the only mind on the planet that could challenge him was Reed Richards. He eventually took control of Reed's mind. But this was Reed's plan and the other members of the Fantastic Four were able to trace him. Thing and the Human Torch attempted to defeat him but his power was too great.
The Invisible Woman sought help but none was available. She resorted to asking Doctor Doom for his assistance. He agreed to help and a huge battle ensued between the remaining Four with Doom and the Over-Mind and Reed under his control. At the brink of defeat, some help arrived in the form of the Stranger. He claimed to be the billions of minds of Gigantus and he was meant to face the Over-Mind. Over-Mind was easily defeated by the Stranger and turned microscopic to rule a dead world of a dust grain. The Watcher tells the Fantastic Four that they were invaluable in helping stop the Over-Mind because the Stranger would only sense him at high power levels. If they had not interfered he may have grown too powerful for even the Stranger to defeat.
Squadron Supreme and the Defenders
The Overmind had been sent to the Microverse. There, Null the Living Darkness found him and decided to use him as a pawn. However, when the two of them left the Microverse, they found themselves in the alternate universe of Earth-S, where the Squadron Supreme lives. They decided they didn't care what reality they were in, and took over Earth, using President Kyle Richmond (Nighthawk) as a stooge. Ultimately, they were defeated by the 616 Defenders and a group of six disembodied telepaths from 616 Earth, led by Mindy Williams, a one-time girlfriend of the 616 Nighthawk. The six telepaths took over the now-empty mind of the Overmind (it had been sucked dry by Null) and lived in his body for some time, even acting as a member of the Defenders for a time. This led to a strange scene of Overmind declaring his love to the Earth-S Nighthawk!
Ultimately Grom took over once again, and returned to his evil ways.