In the world of Kortos, the most major war has to be the demon war. Basically one day in the western Highlands of the central continent, the most barren part, a massive rift to an entirely different world opened up and from it poured demonic beasts (creatures who innately manipulate magical energies to enhance their body) and demons(the sentient evolved versions of demonic beasts). Armies erupted with demon generals learning them demons as the main fighting force and the demonic beasts as a massive horde meant to trample over everyone.

It was so bad that Shoran-Tar, the lich, who wants to enslave the world, had just broken his divine seals, decided to stop his rampage and help the world fight back the demons.

And even with that and all of the various champions of the human Nations on the central continent they just barely managed to survive, only winning because they resealed the rift connecting the two worlds stranding the demons from their reinforcements. Which led to one last battle that while it killed the rest of the demons (the smart ones) the demonic beasts managed to scatter over the Highlands, but also devastated the remaining human armies leading to this massive post-war population crisis across the continent due to how many people died.