After the elders from allied forces slaughtered many demonic monsters and forced those monsters back to the ancient secret realm, several demon elders attacked the other escort elders.
The fight grew bigger by the hour until the main forces from various factions were forced to intervene.
The demons from the Demon Realm were greedy and believed they could team up with the ancient demons from the ancient secret realm.
That caused the people from the Human Realm to feel threatened. They had to fight the demons and ancient demons.
The beasts and demi-humans from the Northern Realm were also caught in the heat of the situation after their escort elders were wiped out by the demons.
That was why the great war suddenly happened just an hour after Yuuki and other powerful experts guarded the realm's portal to prevent any monsters from invading the outside world.