The monsters were freed from the under ground. The humans are racist toward the monsters but the leaders of the human race have decided to restrict any human or monster to harm another to prevent another war.
'When I was 13, I killed someone.
By accident of course...
It was the day I found out I possessed some type of power.'
You kept it to yourself for years until one day it all went wrong.
For your own safety you run away to the depths of Mt. Ebott in hope that the legend was true.
But once you fell down you start to realise that this may not be the first time...
"You've been here before, Y/N. And this time I won't let you run"