We're starting to see what lead up to it, at least.
One of the things I like about this series is how well it captures the dark futility of the Asian Earthbound afterlife(not counting those that ascend to heaven or hell, etc).
Human beings with spiritual energy are the constant target of spiritual creatures, and are often forced to fight these creatures, which leads to lives of hardship, pain, and loss... which inevitably make them prime subjects to become powerful demons and spirits, bringing the cycle full circle.