.
.
.
From what I can recall, the Guardian said something about the "Vile Miasma" which was eating away the Land all around of life. This is why the Land of the Demons, which was the most affected by the Vile Miasma had grown unfertile. Only monsters roamed around and there were apparently dangerous dungeons where even more monsters came from. The Demons, once a peaceful conglomeration of tribes and small nations had now unified below the same banner and led by the Demon King to conquer the Fertile Lands, mostly governed by the Humans and the Elves.
Check "The Protagonist's Sister Is Actually the Strongest"!