While both Gaia and Syndel created Medius, the Medians gave Gaia all of their praise, while Syndel was shunned. Syndel did not take this kindly, and sent his Demons to attack the Medians. Gaia took notice and sent her Angels to defend the Medians. And so began the long series of battles which became known as the Great War.
As an initiating act for the war, Syndel aided an unnamed general from an unnamed kingdom (whether that kingdom was Nirveus or not is still unclear) into taking over his own kingdom. While invading another kingdom, witnesses claimed to have seen Demons within their ranks. As a result, Gaia sent an army of Angels to do battle with Syndel's forces.
Sometime prior to the end of the war in Medius, a band of Heroes took part in the war in order to end the conflict. One hero in particular, Valfa, is mentioned on the back of a book called "The Great War". From the text that is readable he/she seems to be considered a legend.
During the war, a native Median created the Rainbow Reverie, using bodily materials from various races including the Angels and Demons. While these potions could have been used by Medians fighting the war they could have also been used by the Heroes who helped end it for the Medians.
Sometime during the war, hundreds of years after the main conflict on Medius had ended, at least two known clans were massacred by Demons. One of them belonged to Kieri's mother, who would be the sole survivor and later married into the Suizahn Clan; while the other was the Denevol Clan in which only Denevol was the only known survivor.