Fuhai

Fuhai (腐敗) can best be understood as the "corruption of decay" which is thought to be caused by the overlap of underworld energies into the land of the living. This is not just physical corruption, but moral corruption as well. Anyone who spends more than a very brief period in lands strong in Fuhai will begin to suffer both physical decay (living decomposition or putrefaction, mutations, deformities), and mental decay (derangements, inversion of morality, insanity). The longer and more intensely one is corrupted, the more difficult it will be for her to become purified. The same is true of Fuhai-saturated land.

Geographic Instances of Fuhai
The exact location of Aranami's escape from the underworld (which caused a huge, physical fissure that parted the veil between worlds) is unknown and presumed to have been covered over by the shifting of the earth during the Tenpenchii. Since then, however, there have been many other instances of these fissures and weaknesses of the veil. It is even considered that the original fissure, the Great Maw, (Daiwarukuchi 大悪口) remains beneath the earth, since it was never properly sealed.

The Thin Veil, Usuginu
Literally, "sheer silk" or, "the veil of mystery," Usuginu (薄絹) are places where the metaphysical wall between worlds has weakened (be it a thinness to Reikai, the spirit world, or Yomi, the underworld). When instances of Usuginu lead to Reikai, Terrestrial Kami find it easier to affect the living world, possess people, or manifest themselves bodily. Similarly, when instances of Usuginu lead to Yomi, the same is not only true for Abyssal Kami, but also for ghosts and other insubstantial Youkai (Abyssal Kami rarely take advantage of such things, however, as they are largely unconcerned with the land of the living).

Instances of Usuginu that lead to Reikai are the less dangerous of the pair (though they are not by any means "safe"), and typically manifest in an elemental manner that depends on whatever Kami are in the area. So if a fissures happens in an area where Fire Kami are plentiful, the area may become hot, people may become particularly passionate, spontaneous combustion may become an issue, and so on.

Instances of Usuginu that lead to Yomi are considerably more dangerous, as the area immediately begins to become strong in Fuhai (to greater or lesser degrees depending on how weak the veil is). If the veil becomes so weak as to rip open, an Abyssal Fissure is formed.

Abyssal Fissures, Warukuchi
Warukuchi (悪口), literally, "wrong, bad, or evil opening, mouth," is a physical hole in the world (usually manifesting as a hole in the ground, or a cave in a cliff, or a doorway in a home that leads to nothing but darkness, etc.) that feels viscerally wrong. While the physical hole may not be very deep, should one step into an Abyssal Fissure, one is immediately transported to some location in Yomi that may have no corresponding geographic similarities whatsoever, and so it becomes very easy to get lost. Fuhai is simply a matter of course in Yomi for the living body, but the area around an Abyssal Fissure because saturated with it far and beyond an area with Usuginu.

Simply destroying the physical manifestation of the Warukuchi will not remove it. If that doorway is destroyed, the fissure will reform in the nearest portal (even if that portal is merely a small hole in the ground). If there are no open portals, the fissure will reform as soon as there is one, and continue to leak copious Fuhai in the mean time. Only purification rituals, magic, or charms can permanently close a fissure and restore the veil.

Blasted Lands, Koshitokoro
Once an Abyssal Fissure has existed for an amount of time substantial enough to have killed or corrupted/mutated all local flora and fauna, the place becomes known as "blasted" or Koshitokoro, "a blasted place" (枯死所), a place of withering and death. These locations have many other names, as succinct as "deathlands" to the more metaphorical "shadow lands" or "land of corruption" (fuhai no kuni). Fissures also come to be called this if Oni or Youkai begin to come through the fissures and establish a beachhead.

Jade
Jade and Jade-inspired magics (or Sun-inspired magics, calling on Amaterasu) are the most common methods of protecting against or dealing with Fuhai. Jade as a mineral wards those who carry it against corruption and darkness in general, but Fuhai specifically, as the Jade Dragon is a representation of harmony and purity, and has little tolerance for spiritual incursions that muck about with the natural order of things. Similarly, Amaterasu's purifying light tends to burn away corruption.

Onyx
Confusingly, perhaps, and less known (and appreciated) than Jade, Onyx is an element that wards against Fuhai as well. Scholars continue to debate why this might be so, as many would expect the Onyx Dragon to be aligned with Fuhai, given his proximity to the underworld. Perhaps it is this familiarity that allows Onyx to repel Fuhai--the corruption sees its own and leaves it alone. Perhaps not.

Crystal
To a lesser extent than jade or onyx, crystal can be used less as a ward again Fuhai and more as a weapon. Metaphysically, crystal is a physical manifestation of purity that contains only Amaterasu's cleansing light within itself. More practically, forging crystal of sufficient purity to be useful (and in such a form as to be useable) is often outside the realm of mundane craftsmen. More often than not, however, any given temple or shrine that has had to deal with ghosts and the like frequently has at least one crystal ritual blade for such things.

Salt
A more common and less powerful answer to Fuhai than any of the above, salt is also known to be a purifying element that will ward against both Fuhai and Youkai but only in so much as it acts as a boundary both generally avoid.

Death Taint
The Taint behaves much like an infection or disease, spreading through contact and infestation, but it possesses a will and purpose that no real illness can match. As the direct manifestation of Yomi, its goal is to corrupt everything it touches, and it subverts the mind and soul just as readily as the body. And unlike a normal disease, the Taint is almost impossible to get rid of – once its touch has corrupted something, it remains forever after, growing steadily and eating away at the purity of its victims. Living creatures infected by the Taint succumb to strange mutations, exhibit unnatural strength and eerie powers, and become violent and malicious as the Taint subverts their thoughts and dreams. Corpses suffused with the Taint re-animate as undead abominations, seeking to destroy everything around them.

Mechanically, Taint infection is represented by Ranks and Points, similar to the way in which Glory and Honor are tracked, with each Rank divided into 10 Points. When a character accumulates 10 points of Taint, he gains one Rank and his points reset to zero. However, whereas Honor, Glory, and even Status can go up or down as events dictate, Taint generally only goes in one direction: up.

If you happen to find yourself Tainted, the wizards will be sure to inform you of any effects you need to be aware of.