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.
Poseidon was the son of the Titan deities Chornus and Rhea, he was also referred to as Neptune. Poseidon sided with his younger brother Zeus in slaying their father and freeing their siblings from a prison in the dark underworld known as Hades. After defeating their father, Zeus became ruler of Olympus and he granted each of his sibling’s dominion over the regions of the Earth. 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.
With the exception of Poseidon, who still actively watches over the Atlantean peoples, Zeus and the Olympians gradually withdrew from Earth over the centuries. Poseidon and Ame-No-Mi-Kumari combined their powers to protect the Earth from the demonic Japanese god of the underworld known as Mikaboshi. Together they flooded Mikaboshi's underworld, defeating him.