Nemesis has an origin shrouded in mystery. It is said that she was the the sentient being that created the Infinity Gems, it is also said that she might be older than the current universe. At a moment in her history she committed a form of suicide and transformed herself into six very powerful gems, the Infinity Gems. Each of the gems possessed an incredible and different power, which the wielder of that gem would acquire.
The vampire known as Rune tried to reunite the Infinity Gems into the entity known as Nemesis. He traveled to the Marvel Universe to take possession of the Infinity Gems that were in the custody pf the Infinity Watch. The Silver Surfer was sent to battle Rune and prevent Nemesis's resurrection by the Living Tribunal. The Silver Surfer managed to blast one of Rune's hands in which he held five of the six Infinity Gems. The last Infinity Gem was the Soul Gem and was on Rune's forehead. The five Infinity Gems were scattered throughout the Ultraverse.
Soon after Loki traveled to the Ultraverse and gained possession of the six Infinity Gems. Then the Elder of the Universe Grandmaster came and challenged Loki to a bet, where they would have to wager their Infinity Gems. Loki with his six Infinity Gems and the Grandmaster who wagered his seventh known as the Ego Gem (which was possessing the Eternal Sersi). Both selected teams from both the Marvel Universe and Ultraverse. The Grandmaster picked the Avengers from the Marvel Universe, while Loki picked the Ultraforce from the Ultraverse. Eventually Loki won this interdimensional wager by using his Reality Gem.
When Loki and Sersi came into contact, the seventh gem fused with the other Infinity Gems, recreating the original form of the Infinity Gems, Nemesis. As the living embodiment of the Infinity Gems, Nemesis could achieve virtually anything just by simply willing it.
She was weakened by the sentience the gems had achieved during their time as individual entities and Nemesis was defeated by the combined efforts of the Avengers and Ultraforce.