The "hidden realm" is where Spirits and Demons live.
The "Mortal Realm" is where humans live.
And then there's the "Void Realm" at the bottom of the earth...

The Void Realm is a dumping ground for the "impurity" that flows out of the Mortal Realm.
Long ago, in the Hidden Realm there was a sacred tree called the "Divine Spirit Tree",
which was tasked with guarding the entrance to the Void Realm.
One day, it fell seriously ill, and a great deal of impurity overflowed into the Hidden Realm.
The Guardian God (The God of Protection) severely lamented this and died to block the entrance to the Void Realm, but
the impurity was scattered into the Mortal World as "Demons."
A certain herbalist begins his journey to return the demons to the Divine Spirit Tree.