Looking at the options in the DMG both the Necromancer and Oath Breaker fit perfectly in to Oriental Adventures. However, that only covers Tainted and fallen Sohei (Paladin) and Shukenja (Cleric). Here is an arcane caster option and the Maho-Bujin works for non-casters.
When using the villainous options from the Dungeon Master’s guide add the Maho spells to both the cleric and paladin. They otherwise remain unchanged. You will need to adjust the class feature level depending on if the base class is the wizard or sorcerer. Example below uses the sorcerous origin progression.
Bloodbenders and Bloodspeakers
Maho, or blood magic, is perhaps the most seductive and effective tool of evil in Rokugan. Maho wielders, or maho-tsukai, have served the will of the Shadowlands for centuries, both within the empire and without. While the Kuni witch hunters, Phoenix inquisitors, and Snake clan magistrates have dedicated themselves to purging maho from the empire, it is often those most dedicated to its eradication who are most likely to find themselves seduced by its power. And it is power, most often, that seduces, for maho is an easy path to tremendous magical power.
Young shugenjas frustrated at the slow pace of their studies, old scholars chafing at the limitations of their elemental magic, peasant folk-magic practitioners — for all these spellcasters, maho offers a way to overcome obstacles that the sanctioned magic of Rokugan cannot provide. At the mere cost of one’s soul. . . .
Maho-tsukai come from a variety of backgrounds. Many are shugenjas — some inexperienced, some quite advanced — who stumble across or seek out the forbidden secrets of maho in their school’s libraries. Others are adepts from the peasant classes who learn maho from an older practitioner.
Many Bloodspeakers fall into this category, passing the secrets of maho down from generation to generation. Characters who learn maho without prior magical training usually begin as sorcerers.
Beginning when you choose this origin at 1st level, you gain the Shadowlands type (which means you are affected by Shadowlands and Taint specific spells). You are also vulnerable to jade weapons.
By definition, a mahoutsukai is overwhelmed with Taint, but this Taint is not immediately obvious to onlookers. After you become a mahoutsukai, your Taint manifests in mental effects — insanity, rage, nightmares — and internal physical effects. If a mahoutsukai is split open, the internal corruption is obvious: Your body is full of bloody pus and strange growths, even internal limbs. Barring this condition, it is impossible to detect a mahoutsukai based on appearance.
Finally, you no longer apply your Taint score as a penalty to your Constitution, but still applies half of your Taint score as a penalty to your Wisdom.
Starting at 1st level, rather than using a key ability score for spellcasting, you use your Taint scores. It otherwise acts exactly like your original casting ability previous to becoming corrupted – Intelligence for wu jen, Charisma for Sorceress and Wisdom for Divine spellcasters. You also add the Blood Element spells to your list of spells.
Additionally, you must substitute a drop of blood for a spell’s material component or focus, if any. You have two options: use your own blood or use someone else’s. If you use your own, the minor knife cut to draw the requisite blood is part of the Cast A Spell action that becomes a normal part of casting the spell. You do not need to make a Concentration check to complete the spell, despite the wound. Using this ability deals you 1 point of damage. Substituting blood for a costly material component requires drawing a greater amount of blood. You deal more damage to yourself when doing so, according to the table below. When using blood to replace a costly component, you must make a Concentration check (DC 10 + damage taken or spell level whichever is higher) to successfully cast the spell.
Component Cost (gp) Damage Dealt
Starting at 6th level, you may use blood to enhance your spells. You may voluntarily take 1 level of exhaustion to increase the spell level of the of any spell that allows it to be cast at a higher level without actually using the higher level spell slot.
At 14th level, your mastery of blood magic allows you to more easily control other creatures that have blood. Any spell that dominates, charms, paralysis or in some other way immobilises the target are much stronger. The target creature has disadvantage on the saving throw to resist the ability.
Beginning at 18th level, you may use 30 hit points of blood to maximise a spell of your choosing – all its variables are at maximum.