The Great Demonic War was a massive invasion of demons involving the Demon Emperor Infernum, his lords, and his armies from a spatial rift in the Northern Mountain Range. In the span of a decade, almost all of the nations of the Akora continent were destroyed.
The native gods, having become desperate, summoned the 'Summoners' which are the name of foreign humans from another world.
The Aeternum Empire was a regular nation on the coastline, but was lucky to be the last surviving nation. Thanks to this, the ideas from the Summoners allowed enough time for the Aeternum Empire to undergo a massive industrial revolution. The nation's economic, industrial, and military strength underwent a massive transformation that allowed them to gradually push back the demonic armies.
During the three-hundred years of devastating war, the Aeternum Empire gradually retook the lands and forced the demons into desperation. At the height of the war, the Demon Emperor Infernum and the demon lords were forced to fight. However, they were easily defeated by a mysterious 'hero' who disappeared shortly after.
The curtains of this long war gradually closed as the demons retreated into the mountains and into their rift. The demons left behind were either hunted down or forced to go into hiding as the Aeternum Empire sealed off the spatial rift for good.