Five hundred years ago, a war broke out between all races—the demons, humans, elves, beast men, and dwarf races. They hated each other and fought each other for their people and lands. All of the races have the same mindset: that they are superior beings, and no one can win against them.
The human kinds have 10 chosen people by gods; they have the power of the gods and they are known as "demigods," elves have the power of the "world tree," beast men have their superior physical strength, and dwarfs have their enchantment weapons, but demon-kind was superior to them all, and even though all of the races combine their power, they won't even hold a candle against demon king "Satan."
The war has been going on for years. After lots of slaughter and casualties, All races have come to an agreement that they will stop fighting each other and live in peace, but demon king "Satan" is against it. He knows that he will win the war, but after looking at his people and the casualties his kind has suffered, he agrees with it. Satan was a ruthless being; he doesn't give a damn about other races, but he is a soft-hearted person for his people; he loves and cares for them. So he didn't want to see his people being slaughtered before his own eyes, so he agreed.