Ōyashima, as the island is known by its natives -the Nihon, literally means Eight Emerald islands in the language of the Celestial Heavens.

The Nihon's society is based on a clan structure, with Seven Great Clans, as well as Several Minor Clans. Each clan has areas of land bequeathed to them by the Emperor, which they are tasked with maintaining. The Emperor retains ownership of all lands, however, and the clans essentially rent the lands they use by paying taxes annually.

The Great Clans were each formed by one of the Sojin (angels), after their fall from Tengoku (heaven), and their histories date back to the very beginning of the Empire.

Each of the Great Clans is made up of several family lines, and those bloodlines have their own general purpose within each Clan. Each Clan also have a specific purpose within the Empire -- some area of expertise or mandate that specific Clan can fulfill better than any others.  And each land reputedly holds a shard of the original shape of the world in crystalline form (the Maelstrom).

Yashima - Myths and Legends
In the Beginning, there were the Five Races who determined to combine their will to make a world that would last forever.  With their will, they would replace the maelstrom of energies from which the world was actually composed. The first five kami, the elemental spirits, create everything commingling themselves, using a big crystal as focus located in the City of Night. The newborn lesser kami drew lines of order through chaos, formed boundaries between earth and sky and between land and sea. They gave purpose to fire's hunger and made the Void into the balance between all and nothing.

Ningen-do, the mortal realm as man knew it, was created. The composed of a balance of the Five Elements, as well as the influence of Thunder in mortal souls.

The creation of this new world awakened the dark realm of Jigoku, which looked upon the stability of Ningen-do with a terrible hunger. It wished to destroy this newborn world, to feed upon the kami and enslave the Five Races to its service. It sent forth its servants, the ruthless oni, and began to wage war upon Ningen-do.