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Backstory from the Game Guide
The Creator created the original mankind, "true people" (Arcanians), to the semblance to himself far long time ago. That is to say, the word "man" originally meant these "true people".
True people's civilization got remarkably developed by using Crystals, but this development was accompanied by a loss of their psychic equilibrium. Because the power of the Crystals originally comes from "Chaos". One can get a huge power with using "Chaos" but that results in considerable increase of his own internal chaos.
As a corollary, people in this age came to perform diabolical experiments again and again.
That is a truth of the Golden Age.
The Seven Wonders are the establishments for those experiments.
The influence of the Chaos was worldwide, caused violent weather and brought forth a lot of creatures that were not seen before that, such as the undead, dragons and also human. In this sense, monster and human are equal.
As the Creator was deeply concerned that the whole world might be swallowed by the Chaos, decided to terminate the most civilized region that was a frontier of studying Crystal.
The world was devastated, the rugged mountains and deserts appeared on the surface of this planet.
The act of destruction also degutted the Creator.
He disappeared from this world, then came "the age without God".