Yōkai

Yōkai (妖怪?, ghost, phantom, strange apparition) are a class of extremely chaotic, corrupt demonic entities found in the world. They hail primarily from Makai - with countless, perhaps billions of Yōkai kind being available in that distinctive realm. However, the Yōkai race has began to become far more common in Earth as of late, given the influx of chaotic spirits, as well as the after effects of Ratoka's Gehenna Assimilation Ploy.

Origin
For millenniums, Yōkai were a rare but known phenomenon in the world. Often times, they were spawned from the deceased, with vengeful humans evidently being unable to pass on from this world, and enter a new peaceful life in the next world. As they remained, their soul festered, eventually becoming a corrupted spirit, and subsequently devolving into a Yōkai.

These dark creatures often terrorized their localized area, often requiring the need of either a shinobi, shaman, or exorcist in order to truly vanquish. Nevertheless, in time, the fiends began to grow more and more common - with nine titanic beasts, rumored to have been spawned from the powerful souls of Ninja Monks and Priests, eventually rising into power, as the fearsome Tailed Beasts.

But even with the emergence of these magnificent animals, human vices and the evil that naturally emerged in the hearts of men did little to quell the growing threat even further. Thankfully, in spite of the terror that was caused by these monstrous villains, Demons still remained an anomaly or minority to the world at hole - with the vast majority being trapped, locked away within the dastardly realm of Makai.

Expansion
With the Dark Sorceress, Ratoka, to use the power of several corrupted Gelel Stones to rip open a permanent portal linking the Human Realm with the Demon Realm, the expansion of demonic entities along the globe skyrocketed. While her plan ended in an embarrassing failure, the partial activation of her dark spell would forever shift the balance in the world. As a result of her ploy, the spatial symmetry of the Human Realm had forever been altered, allowing rips in the space-time continuum that would go on to serve as breaches that demons of varying classes could slip through.

The bulk of the spatial breaches were located in the Continent of Chusei - primarily in the appropriately named Land of Demons - as well as the Land of Wind. Within a year's time., the number of Earth's Yōkai had tripled, and after half of a decade, the Yōkai infestation had become a permanence of Earth, with the demonic creatures now being available in all walks of life.