The Old War
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"What happened back then was when your parents had yet to become heroes, in the previous war against the demons, the prelude before the Demon King were to be born. The ancient Demon Lords that served the Demon King sent armies to this continent, and the many Kingdoms had to defend their capitals against the endless armies of demons, even the wilderness of our jungles were not untouched by the malice and darkness of these beings…" said Ninhursag.
The Demon King didn't existed forty years ago? Well, my father is not that old, I think he's not even thirty years old yet, so it makes sense, their journey began when he was around twelve, so the Demon King was only born almost twenty years ago… a bit less than that.
Meanwhile, Ninhursag is over forty years probably.