In the early ages of the Earth, a sentient life-force developed and one day manifested itself into the being Demiurge. He birthed the Elder Gods who eventually warred among themselves until one of them, Atum, transformed itself into Demogorge and devoured or chased away all but himself and Gaea, also known as Mother Earth. Atum would reform from the Demogorge and leave the Earth, dispersing the energies he had absorbed into the planet. When mankind eventually began rising towards sentience, their dreams gave form to the absorb energies; this transformed those who would become the progenitors of mankind's gods, notably the Olympians, regarded as deities by the peoples of Greece and the Roman Empire.
Mythology holds that Gaea, the sole remaining Elder God, gave birth to Ouranos. She then mated with him, and they parented the Olympians, the Titans. They resided in an interdimensional pocket universe in the city known as Olympus, though the City's origins are unrevealed. Ouranos was eventually slain by his son, Chronus, who feared the same treatment from his own children and he began imprisoning them in a dark underworld known as Hades. Before Chronus could similarly banish his sixth child, his wife Rhea fled through a dimensional portal to Earth, arriving near Greece's Mount Olympus, where the child was raised among the shepherds. After reaching adulthood this child, Zeus, freed his siblings Demeter, Hera, Poseidon, Pluto and Vesta from Hades and led the war against their father, a war they would eventually win. Zeus imprisoned most of the surviving Titans in Hades and set himself up as ruler of Olympus, taking his sister Hera as his queen. Pluto was asked to rule over the underworld and guard its prisoners, Poseidon was given dominion over Earth's seas, Demeter was assigned to watch over the land, and Vesta was to watch over the people.
Zeus ruled a peaceful Olympus for centuries while he and his siblings populated the realm through interbreeding with humans, extradimensionals, Titans, and whatever else caught their fancy. Select mortals or aliens were occasionally added to the pantheon as well, sometimes further empowered by Zeus.