Prologue
Long ago, two races ruled the earth: Humans and Monsters. One day, war broke out between the two races. After a long battle, the humans were victorious.
They sealed the monsters underground with a magic spell.
Why did the humans attack? Indeed it seemed that they had nothing to fear. Humans are unbelievably strong. It would take the soul of nearly every monster, just to equal the power of a single Soul.
But humans have one weakness. Ironically, it is the strength of their Soul. Its power allows it to persist outside the human body, even after death. When a human dies, its soul remains stable outside the body. If a monster defeats a human, they can take its Soul. A monster with a human Soul… A horrible beast with unfathomable power.
The power to take their Souls. This is the power that humans feared. This power has no counter. Though monsters are weak, with enough human souls... They could easily destroy all of mankind.
Indeed, a human cannot take a monster's Soul. When a monster dies, its Soul disappears. And an incredible power would be needed to take the Soul of a living monster.
There is only one exception. The Soul of a special species of monster called a "Boss Monster." A Boss Monster's Soul is strong enough to persist after death, if only for a few moments. A human could absorb this Soul. But this has never happened. And now it never will.
Hurt, beaten, and fearful for our lives, we surrendered to the humans. Seven of their greatest magicians sealed us underground with a magic spell. Anything can enter through the seal, but only beings with a powerful SOUL can leave. There is only one way to reverse this spell. If a huge power, equivalent to seven human SOULs, attacks the barrier... It will be destroyed. But this cursed place has no entrances or exits. There is no way a human could come here. We will remain trapped down here forever.