Humans and spirits once coexisted, along with other sentient beings. Led by the Spirit King, the spirits can physically jump between the mortal land and spirit Realm. Humans, on the other hand, have souls that can reach the Realm.
Humans, unlike spirits, can be vessels for the dark creatures known as Corruptions. Corruptions have found their way to all human hearts.
Their parasitism led to humanity's greed, jealously, hatred, and madness.
Humans concluded to siege the spirit realm for power. The war between humans and spirits thus began.
Humanity was able to learn--with the thrive from the Corruptions within them--the spiritual tactic to combat the spirits: magic.
Humanity was led by the strongest individual to yield magic. She was known as the Witch.
At the height of the war, the Spirit King was able to confine all of humanity in a space within the Realm. There, he fought the final battle with the Witch.
But the witch was no pushover.
Before falling to the Spirit King's judgement, she willed out her power to create a curse that made the space they're confined in to become the Boundary that separated the Spirit King and the Realm to the mortal land.
After years of being trapped in the Boundary, all humans were taken over by the Corruptions.
The spirits that were left behind in the mortal land needed to return to the spirit king in order for him to gain full power in removing the Boundary.
Spirits have attempted to cross the Boundary only to succumb to the Corruptions.
Longing to still reach the Spirit King, the spirits would turn themselves into Flutters before dying in order to escape the Boundary and return to the mortal land. In that form, they are weaker and fragile but as long as they live, they can eventually provide power for the Spirit King.
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The story starts with Hevi, a spirit determined to form a group of spirits that will finally reach the Spirit King after centuries of his kind failing to do so.