Oni are a race of evil spirits, native to the Material Plane, that manifest physical bodies based upon the shapes and desires of humanoid mortals. In pure spirit form, an oni is nothing but a disembodied evil longing for the sins of the flesh. In this form, oni are harmless and invisible. The majority of these bodiless oni were once kami who failed their wards, or more often, who deliberately abandoned them. As punishment, they were stripped of their ability to form a physical body and then cast into the void. Rarely, a mortal creature's soul can become a disembodied oni upon death, or in even rarer cases, after a truly evil individual has undergone a particularly vile ritual that ends in suicide. These oni are more often destined for positions of great power and strength than most. Eventually, an oni's spirit manifests a physical body on the Material Plane. The methods by which this may occur vary, but the process generally takes place in areas already despoiled by sin, tragedy, or cruelty. The type of oni a spirit transforms into is influenced by a wide variety of variables, ranging from the nature of what the oni spirit was before to the location where it is reborn into the realm of flesh and blood. Once an oni manifests its physical body, that body becomes its true form for the rest of its life—all oni are shapechangers, but this original form is the form in which they are born and the form they revert to upon death.
An oni's true form is always similar to that of a specific type of humanoid, save that it is always deformed and monstrous to look upon. Tusks, additional eyes, or strangely colored skin are common physical attributes. Yet while they have hideous forms, all oni are capable of changing their shape to assume forms more pleasant to the civilized eye. The type of shapes an oni can assume depend upon its species, but all can transform into some form of humanoid. Oni use this ability to infiltrate humanoid societies, either to prey upon the weak or to rule them in disguise.
In order to understand oni, it is important to consider that most of these beings were once spirits tasked with protecting a material realm they now believe is undeserving of such care and concern—indeed, the physical world is to be dominated and consumed. They arrive in physical form starved for sensory experience, and never fully sate their desire to gorge on such experiences. Most oni seek to attain positions of leadership and power, often in the guise of a normal humanoid, in order to secure a never-ending supply of sensory experiences. It is also important to understand that oni's immoderation is also motivated by their corrupted and evil natures—that is, no pleasure is more enjoyable than one that deprives or wounds another.
Oni retain their hatred of the kami upon reentering the Material Plane as physical creatures, and often their depredations and violations of the world can be directly tied to their efforts to destroy kami or the kami's works. Oni tend not to congregate with others of their kind, being most comfortable in positions of leadership over enslaved or oppressed societies populated by humanoids they can masquerade as. Yet, at times, particularly powerful oni abandon this mindset and instead gather to their side entire legions of oni drawn from all types. The oni known collectively as the yai—oni with an affinity for true giants—are most often responsible for such actions, and when a yai builds such an army of oni, the humanoid lands shake with terror.
Theoretically, as many types of oni exist as there are types of humanoids, although in reality, certain types of oni are much more populous than others. The ogre mage, an oni associated with ogres, is the best-known and most commonly encountered type of oni, for reasons that still send scholars of oni lore into spiraling arguments with seemingly no end. Beyond the oni detailed on the following pages, the atamahuta (ettins), ja noi (hobgoblins), nogitsune (kitsune), and wind yai (cloud giants) are relatively well known. Oni with associations to bugbears, great cyclopes, stone giants, trolls, troglodytes, and other races exist as well. And above even the mighty void yai there exist the oni demigods—entities of nearly incomprehensible power known as the oni daimyo.
Countless oni stalk the world, slaking their greed and bloodlust to the misery of humanoids. The great leaders among the oni, beings known as daimyo, are as numerous as the nations of the world, and all oni seek to climb the rungs of power to become such powerful entities. The following list includes oni daimyo that have power in ways that make them known and feared outside their own domains. Most of these nefarious oni command armies or control points through which other oni can enter the world.
- The Horned King (ogre mage)
- Hunter of Blood (ogre mage)
- The Sea Devil (water oni)
- Empress of the World (void yai oni)
- The Dark Mother (ogre mage)
- The Red King (fire oni)
- Dancer in Flesh (ogre mage)
- Keeper of the Oni Gate (void yai oni)
- Daimyo of the North (kuwa oni)
- Lord of the Caldera (fire oni)
While oni are incredibly varied in their shapes and powers, they still share certain traits in common.
- Regeneration: All oni regenerate, although the rate of regeneration varies. Damage from fire or acid can kill an oni.
- Change Shape: All oni are shapechangers, but their ability to change shape is generally limited to creatures similar in shape to their true form.
- Native: Oni are native outsiders. As evil spirits clad in mortal humanoid flesh, all oni also possess one of the humanoid subtypes—usually giant.