Updated and reprinted here for 5e from 3e – though its mostly for my players reference. Still a work in progress as not sure ability damage is the best way to go for the Taint but its otherwise done.
Beyond the great Kaiu Wall and the Twilight Mountains in the lands of the Crab clan, a vast, desolate wasteland stretches for uncounted miles. No plant grows that is not corrupted by evil, twisted into a flesh-eating monstrosity. No water runs that is not contaminated and foul. No breezes blow that do not carry the stench of rot and decay. Even the light of Lord Sun must filter through a miasma of noxious vapours and cloying fogs before reaching the earth, pale and dim. And with the exception of the nezumi, no creature dwells in the Shadowlands that is not utterly, irredeemably evil.
One legend says that the Shadowlands resulted because Lord Moon’s blood landed there, but no tears of Lady Sun fell to balance the land. Other think the Shadowlands was created when Fu Leng, one of the nine children of the Sun and Moon, fell to earth after Hantei cut him and his siblings out of their father’s stomach (see The Children of the Sun and Moon).
The impact of Fu Leng’s fall created a deep pit that led into Gaki-do, the realm of evil spirits, and evil welled forth from the pit. The evil of Gaki-do twisted and corrupted Fu Leng himself, and he became lord of the evils of the Shadowlands just as his brothers and sisters became rulers of humanity.
As humanity flourished and spread, forming the Empire of Rokugan, Fu Leng created minions for himself and hurled them against the fledgling empire. Had it not been for the intervention of Shinsei, Fu Leng and his armies might have conquered all Rokugan, but Shinsei and his Seven Thunders succeeded in binding Fu Leng, containing his power in the twelve Black Scrolls.
Though bound, Fu Leng was not powerless, and he made his influence felt even through the “Thousand Years of Peace.” The practice of maho, in particular, caused Fu Leng to stir, and he certainly was able to work his will through maho-tsukai minions such as Iuchiban and his lieutenant, Asahina Yajinden.
With the opening of the first Black Scroll by Yogo Junzo, Fu Leng’s full power began to return to him, and he reached out to corrupt the weak young Hantei who sat on the throne in the wake of the Scorpion Clan Coup. By the time Hida Kisada strode into the throne room in Otosan Uchi to challenge the young Emperor, Fu Leng’s control over Hantei was nearly complete, and a single Crab champion was no match for him.
It took the combined efforts of seven new Thunders, led by a descendant of Shinsei, to destroy Fu Leng once and for all. The death of Fu Leng did not end the menace of the Shadowlands, however. Ogres, bakemonos, and oni still walked the land, and oni lords like Kyoso no Oni provided leadership for these hordes. The corrupted Moto still rode the plains, and shugenjas within the empire continued to fall prey to the seduction of maho.
With the march of armies through the Shadowlands toward Volturnum and Oblivion’s Gate, however, the Shadowlands hordes suffered great losses. Huge sections of the Shadowlands were virtually depopulated. However, while the empire nursed itself back to health and spent itself once more in the Spirit Wars, the Shadowlands also regained its lost strength—and more. Mightier creatures moved into the depleted regions of the Shadowlands, closer to the empire than they had been before and thus a greater threat.
A stronger, fiercer breed of bakemono exterminated its weaker cousins. Mighty ogres with a grasp of tactics took control of these bakemono and their lesser ogre kin. Now, a number of “oni overlords” have appeared, giving even Kyoso no Oni cause for fear — and one of these new oni has taken the life of Rokugan’s Emperor. If the hordes of the Shadowlands have their way, no Emperor will ever again sit on the throne of Rokugan.
The Shadowlands show some diversity of terrain—from the wasted badlands nearest the Twilight Mountains to stinking bogs, twisted forests, and broken mountains. The River of the Last Stand forks in Last Stand Plain, winding south into the Shadowlands and splitting again, forming the Black Finger River, which flows into the Forbidden Lake, and the River of the Dark Moon, which loses momentum and becomes a great bog surrounding the Festering Pit of Fu Leng. Even the Sea of the Sun God is tainted and vile off the eastern coast of the Shadowlands, where the land rises in charred peaks and belching volcanoes.
The Shadowlands is an outer plane in Rokugan’s cosmology. It is finite, clearly bounded on all sides by the Material Plane lands of Rokugan. Traveling from the Material Plane to the Shadowlands is as simple as traveling over land or across a river—no magic is required. Distances are sometimes distorted within the Shadowlands—a journey that should be only a few miles can stretch out over days, though the reverse is rarely true.
MAGIC IN THE SHADOWLANDS
In the Shadowlands, the elements themselves do not function properly, so it is harder for a divine caster to bend the elements to his will with a spell. Thus, all divine magic is impeded in the Shadowlands: In order to cast a divine spell, the caster must make a Religion check against a DC of 20 + the level of the spell. If the check succeeds, the spell works without difficulty. If the check fails, the spell does not function, though it still counts as a spell cast that day. In addition, the spellcaster exposes himself to the Shadowlands Taint (see the next section), and adds 1 to his Taint score for every 5 points by which she failed her Spellcraft check.
Arcane spells, including all spells cast by maho-tsukai, are not impeded in the Shadowlands — in fact, quite the opposite. All arcane spells cast in the Shadowlands have the Empower metamagic ability applied to them, at no additional cost to the caster.
Empower. When you roll damage for a spell, you can reroll a number of the damage dice up to your Charisma or Intelligence modifier (minimum of one). You must use the new rolls.
THE SHADOWLANDS TAINT
The substance of evil pervades the Shadowlands, poisoning the area with its contaminating presence. This corruption, commonly known as the Shadowlands Taint, affects everything that walks and breathes among it—with the notable exception of the nezumi. All who tread in this dark realm are bathed in its evil, and most suffer horribly from the exposure.
The Taint affects the body, mind, and spirit. It mars the flesh, maddens the brain, and poisons the soul. It causes unnatural growth and infuses the flesh with virulent cancers, eventually transforming its victims into corrupted puppets: tattered husks under the control of the lords of the Shadowlands.
It affects the mind as well as the body, causing terrible madness in its victims. Irritability and aggressiveness escalate into paranoia and delusions, culminating in extreme homicidal mania.
The Shadowlands Taint is measured in a Taint score. An initial exposure to the Taint typically results in a Taint score of 1d3, and accumulation is typically fairly rapid after that initial exposure.
For every 24 hours spent in the Shadowlands, a character must make a Constitution saving throw. The DC is 10, but it increases by 5 for every consecutive 24 hours the character spends in the Shadowlands. If the character fails this saving throw, his Taint score increases by 1.
A character’s Taint score applies as a penalty to his Constitution and Wisdom scores. Thus, a character with a 16 Constitution and a 14 Wisdom who acquires a Taint score of 4 has an effective Constitution of 12 and Wisdom of 10. These penalties reflect the Taint’s impact on the character’s physical and mental health. Characters who embrace the Taint and make use of it (primarily maho-bujin and maho-tsukai) can ignore some of these penalties. See the Villainous Class Options for more information.
The penalties to Constitution and Wisdom are experienced by the Tainted character in a variety of ways, from mild nausea, joint pain, or disorientation to rotting flesh, severe skeletal warping, and irresistible murderous urges. Table 12–1: Taint Effects suggests possible effects, divided into mild, moderate, and severe effects. There is never any game effect other than Constitution and Wisdom loss.
As terrible as the Taint’s effects are upon the living, they are even worse upon the dead. Any creature that dies in the Shadowlands (except for oni) animates in 1d4 hours as an undead creature, usually a zombie of the appropriate size. Crab protocols call for burning the bodies of fallen comrades to prevent this ghastly transformation.
RESISTING THE TAINT
The most reliable defense against the Taint while traveling in the Shadowlands is jade. Hiruma shadow scouts and others who regularly travel in the Shadowlands always carry jade with them. Jade absorbs the Tainted energies that would normally corrupt the character carrying it, slowly darkening and softening as its purity gives way to the corruption it is taking in. A “finger” of jade (a piece of jade roughly the size and shape of a human finger) protects a character for seven days before its purity is exhausted, eliminating the need to make Constitution saving throws during that period. Multiple fingers of jade protect a character for a longer time, but only to a point: Two fingers last twelve days, three last sixteen days, four last nineteen days, and five or more last for twenty-one days. A single finger of jade costs 100 gp.
No artificial form of jade carries the same divine balance of elements as natural jade, so using magic to create jade in the Shadowlands is no protection from the Taint.
If a character’s Taint score ever reaches 10, the character is overcome by the Taint and becomes a slave of evil. Such characters feel an irresistible urge to travel to the Shadowlands, where they are often killed by Crab patrols, traps, or marauding creatures. Those who survive are met by an akutsukai, a semi-human servant of evil, who tests them for skill and loyalty. If they survive this test, they are usually trained as maho-bujin.
- Nausea, vomiting
- Pain in joints
- Skin thickens, cracking and turning leathery
- Skin seeps greasy, yellowish “sweat”
- Eyelid swells, obscuring vision
- Hair goes white
- Phlegmy, wracking cough
- Pale, grayish dead complexion
- Sunken eyes, cracked lips
- Mild paranoia
- Mild hallucinations
- Increased aggressiveness
- Bones begin to warp and thicken
- Black lichenlike growth across skin itches incessantly
- Reddened, burnlike sores and scars
- Eye clouds or blood vessels break, obscuring vision
- Lips shrink back from gums
- Gums swell, bleed, and rot
- Bleeding from eyes, nose, mouth, ears, or lips
- Hair falls out
- Uncontrollable seizures that wrack the body with spasms
- Eruption of painful sores
- Sores ooze blood, pus, foul-smelling ooze, spiders or
- insects, thick pasty substance, maggots, acidic green slime
- Hears voices of evil spirits
- Severe paranoia
- Fits of uncontrollable, disturbing laughter
- Disregard for hygiene and cultural mores
- Flesh of nose rots away, leaving skull-like openings
- Mutated, deformed fingers, toes, leg, arm, head, ear, eye, or
- teeth begin to grow on inappropriate parts of the body, then
- shrivel, rot, and eventually fall off
- Spine twists, back hunches
- Severe warping of skeleton, skull enlarges and deforms
- Great swollen growths on the body
- Lungs eaten away from inside—wet, labored, painful breathing
- Eye falls out, leaving gaping socket that glows with eerie
- green light
- Skin peels off in papery sloughs at the slightest touch, leaving
- red, raw flesh beneath
- Fingers or toes begin to web and fuse
- Irresistible murderous urges
- Reduced to primitive behavior
- Eats inedible or still-living things
HEALING THE TAINT
The healing magic of the water element can reduce the Shadowlands Taint, helping to restore the natural balance of energies and flow of ki within a Tainted individual. A remove disease effect (like lesser restoration) or remove curse spell is the obvious choice, but even these spells are limited in their ability to remove the Taint entirely. Neither spell can eliminate the Taint, but either will reduce the target’s Taint score by 1. A heal spell reduces a character’s Taint score by 3, but never below 1. No other spell, including wish, can reduce a character’s Taint score, except by duplicating the effects of these spells.
EXAMPLE POWERS AND MUTATIONS RELATED TO THE TAINT
Those who embraced the taint were granted certain gifts to make the embrace slightly more appetizing. People with the taint found they were capable of greater feats of strength, agility, stamina or reflexes. For those who wished to mingle in society without being discovered, it was possible to carry a jade sliver corrupted by the taint. This protected the bearer against detection and pure magic. This jade sliver would only last for approximately five days before crumbling into dust.
Those who attempted to draw upon the powers of the taint, either subconciously or willingly, would find there were varous deformities or mental twistings as the price.
These were just minor abilites giving the tainted individual powers above the norm. Learning more powers increased the person’s taint further.
- Above the Elements
- Blackened Claws
- Blessing of the Dark One
- Blood Knows Blood
- Blood Sense
- Calligraphy of Thought
- Child of Darkness
- Chitinous Armor
- Claws of the Kumo
- Darkness Unseen 
- Death Sense 
- Eternal Health 
- Eyes of Hell
- Fu Leng’s Hunger 
- Jade Sense
- Master of Blood
- Master of Shadows
- Mind of Darkness
- Monstrous Strength
- Sense Purity
- Taint Sense
- Uncanny Speed
- Unholy Beauty
- Unholy Stamina
These powers were more impressive than the minor ones, but also caused a greater increase in taint. Major powers could only be gained if the person already had a minor power.
- Beside the Darkness
- Billowing Darkness
- Blood Domination
- Chosen of Fu Leng
- Corrupted Blood 
- Demonic Charisma 
- Father of Lies
- Fu Leng’s Venom 
- Greater Blessing of the Dark One
- Speak With the Voices of the Dead 
- Terror of Fu Leng 
- Unearthly Regeneration
- Undead Strength
Greater powers were terrible to behold and could cause great devastation. They came at a higher price however, severely increasing their taint. Greater powers could only be gained if the person already had a major power.
- Armor of Death
- Beast of Fu Leng
- Beyond the Elements
- Blood Shouting
- Bloodstrike Plague
- Book of Souls 
- Disrupt the Chi
- Drawing Out the Darkness
- Evade the Unworthy
- Feeding on Flesh
- Final Blessing of the Dark One 
- Flight of Darkness
- Outside the Elements
- Protection of the Dark
- Strength of Madness
- Strength of the Dark One
- Thy Master’s Will
Deformities and Scarring
Some of the Shadowlands powers caused deformities or scarring in the person. Deformities made the person appear inhuman, and therefore it was harder for them to conceal their dark secret. Scarring was a major sign of a maho user, which was easier to hide, but very damning if discovered.
SHADOWLANDS MONSTERS AND THE TAINT
Creatures with the Shadowlands subtype do not acquire Taint and are not harmed by it, since it makes up part of their very nature. For purposes of detect Taint, spellcasting, and other effects, a Shadowlands creature is considered to have a Taint score equal to half its Charisma score. Undead creatures add +1 to this number, while fiends add +2. Thus, a Shadowlands skeleton has a Taint score of 3 (equal to half its Charisma score +1), a ogre mage has a Taint score of 7 (equal to half its Charisma score), and a kyoso no oni has a Taint score of 12 (half its Charisma score +2).
Bloodspeakers are members of a cult founded by the sorcerer Iuchiban, the most powerful and fearsome maho-tsukai in Rokugan’s history. Their ultimate goal is to put their master, whose spirit still lives, trapped in his tomb, on the throne of the empire, or—failing that—to put any sufficiently powerful member of their organization on the throne. Now that the throne stands empty, the Bloodspeakers are revitalizing their efforts in pursuit of the goal that has driven them for six hundred years.
The Bloodspeakers are a secret society devoted to the overthrow of Rokugan’s strict social order. They are not dedicated to maho and do not think they serve the evil of the Shadowlands, but maho is an important part of their practices and, in truth, they are simply pawns of the Shadowlands’ evil. The cult appeals to peasants and eta who are dissatisfied with their station in life and the lack of opportunities to better themselves that Rokugan’s society affords them. Participation in the Bloodspeakers offers a form of power that is normally beyond such people—a twisted sort of rebellion against everything the empire holds sacred.
The Bloodspeakers number about three hundred members, mostly scattered through the Lion, Crane, and Scorpion lands. They tend to cluster in extremely isolated locations, where they are not likely to be discovered, or in large cities, where they can hide among the multitudes. The cult is based around small cells, each with five to twenty-five members, led by the most powerful maho-tsukai of the group, or at least the most charismatic.
Most Bloodspeakers do not know anything about any other cells, but one or two senior members of each cell know of perhaps two or three other cells, and contact those cells to coordinate efforts on rare occasions.
Bloodspeaker cells meet to read from Iuchiban’s writings, perform maho rituals to increase the power of the participants, and plan strikes against threats or enemies. Blood sacrifices are a common features of these meetings, but otherwise they vary widely in content and tone.
MAHO AND THE TAINT
One of the most common means of acquiring the Shadowlands Taint is through the practice of maho, or blood magic. Shukenjas, sorcerers and wu jen are often drawn to the study of maho as a way of increasing their magical power, giving them access to destructive spells they would not otherwise know.
A shugenja, sorcerer, or other arcane magic user who researches maho can learn a spell from the maho-tsukai spell list instead of his normal spell list if he makes an Inteligence check against a DC of 15 + the spell’s level. This spell counts against the character’s normal spells known, and if the character is a shugenja, it counts as a spell outside the character’s favored element.
Casting maho spells carries a terrible cost, however. Any character who casts a maho spell (a spell from the maho-tsukai spell list) must make a Constitution save (DC 10 + spell level) or increase his Taint score by 1. The character must make this saving throw every time he casts a maho spell.
If a shugenja or sorcerer has a Taint score of 4 or higher, he can qualify for the maho-tsukai villainous class, and increase his magical power dramatically—at the ultimate cost of his soul.