The corrupted, twisted servants of nature given corporeal form to consume and destroy without end
Oni are a race of evil spirits native to the Material Plane, who manifest physical bodies based on the shapes and desires of humanoids mortals - in pure spirit form, an Oni is nothing but a disembodied evil longing for the sins of the flesh. In this spirit form, Oni are relatively harmless and invisible. However, when given corporeal form, Oni can take an almost infinite number of forms - though this true form is always similar to a specific type of humanoid such as Human or Kitsune, save that it is always deformed and monstrous to look upon, with tusks, additional eyes, or strangely colored skin all serving as common physical attributes. Because of this, their anatomy is exceedingly hard to describe for certain - each Oni well varies depending on the exact form it takes, though the common throughline between all Oni is "Humanoid", meaning that most can be found having at least two arms and two legs, with at least one head atop a central torso. They often possess circulatory, skeletal, and nervous systems(Though such systems might well be many magnitudes more efficient and durable than the norm), and as such typically possess the standard set of organs and internals for the race they appear as.
Oni can take an almost infinite number of forms - though this true form is always similar to a specific type of humanoid such as Human or Kitsune, save that it is always deformed and monstrous to look upon, with tusks, additional eyes, or strangely colored skin all serving as common physical attributes. Their bodies can range in size depending on the humanoid creatures they are mimicking, from the normal sized Kuwa(Human-derived) to the gargantuan Yai that mimic true giants such as Dai-Yukai and Jotun. However, not all the Biological Traits of the Oni come from the humanoid creatures whose forms they take - as creatures born from the failings and corruption of Kami, all Oni have biological traits such as skin blemishes, skin color, bone growths, horns, extra eyes or limbs, or such that hearken back to the Kami they were born from - such that an Oni that was once a giant carp-like Suijin might have gills or fins along their cheeks, scaly skin, or other signs of their former aquatic heritage.
Genetics and Reproduction
The topic of whether or not Oni can reproduce through normal means is a debate which sends scholars of Oni lore spiraling into endless debates of varying intensity - many such scholars claim Oni cannot reproduce normally and must rely on corrupting other Kami or mortals, while others claim that they likely can reproduce normally but simply see no value in it in favor of the sheer twisted enjoyment out of blackening the soul of a Kami or mortal being. However, both sides can agree that Oni are at least capable of the act of sex - for it is one of the basest pleasures they delight at indulging in.
Growth Rate & Stages
Oni do not grow - they appear fully formed and ready to indulge in all of life's hedonistic pleasures.
Ecology and Habitats
Because they can take on most any humanoid form, Oni can thus inhabit most any environment or habitat on Zheng-Kitar.
Dietary Needs and Habits
Oni, despite their existence as outsiders, wear humanoid flesh and thus require and delight in indulging in all the pleasures of the flesh that were denied to them during their previous lifetimes as Kami - meaning that they will eat anything, drink anything, and indulge in anything like the hedonistic monsters they are.
As evil spirits with a close connection to the spirit world, Oni do not age - they only get stronger as they age and conquer and indulge.
Oni do not have any social structure of their own - they tend to avoid others of their kind and favor slipping into the social structures and societies of the creatures whose bodies they mimic the forms of. Only the monstrous Oni Yai, who mimic True Giants, and the terrifying Oni Daimyo, towering Oni who have ascended to Demigod status, dare to rally and subjugate their own kin. However, Oni do have a "code" of sorts they live by - this Oni "philosophy" is built on three pillars: maekaga (dominance), kaedakaga (punishing the undeserving), and tezukaga (hedonism): these are the guiding principles of all oni, with tezukaga taking precedence. Despite their fallen state, oni are still bound by the Laws of Golden Perfection, the ancient laws that bind all Kami - The most important of these scriptures are a ban on open war between oni and kami and forbiddance of direct intervention in mortal affairs. However, oni may still work around these laws by employing non-oni agents.
Oni faces are terrifying, demonic things - they often possess sharp fangs, extra eyes, strangely colored skin, and sharp and jagged features that lend them an imposing demonic visage few can tolerate or stand for long periods of time.
Geographic Origin and Distribution
Oni can be found almost anywhere across Zheng-Kitar, as they appear whenever and wherever Kami fail to perform their duties or live up to the laws that bind them, and are so overtaken with rage and despair that they become an Oni rather than simply fading away. However, for reasons that have only recently become explained, they tend to be most prevalent in the lands of The Narixian Empire, as the legendary Boiling Lake was only recently discovered to serve as a "plug" of sorts on some sort of focal point for Oni - whether that be a giant Oni Demigod(Known as Oni Daimyo) or some sort of twisted Oni Citadel. This "plug", though it was recently discovered to be watched by eternally vigilant Githyanki and Narixians since the formation of their culture, serves as a twisted nexus for Oni, making their appearances much more likely when Kami in the area fail in their duties and begin to fade away or even when Oni crawl out of the lake itself, ready to indulge in mayhem and slaughter.
Oni tend to be quite intelligent on average - they are well-equipped to blend into the societies of the humanoids they mimic the forms of, and often weasel their way into positions of power within such societies to better indulge in their hedonistic appetites with abandon.
Perception and Sensory Capabilities
Oni senses can vary somewhat - the different forms they take means that while most all Oni possess the ability to see in the dark, many others might have low-light vision or even true seeing, letting them see through all magical illusions and fakery.
Civilization and Culture
Oni names tend to titles or epithets rather than names - such as "The Horned King" or "Lord of the Caldera". Their names are typically taken from the conventions of the humanoid species whose bodies they mimic the forms of.
Though the origins of the Oni vary and are hotly debated, a common theory prevails that they were in fact formed from some of the very last surviving Kami who endured the total obliteration of the continent at the hands of the Yema as they sought to Defeat the Sapphire Elves thousands of years ago, before even the Advent of the Spirit Kings. Consumed with rage and despair as the continent burned around them, rendering them unable to fulfill their roles as guardians of the natural world, these last Kami, rather than fade away as the Yema burned away their wards and homes in great sheets of divine fire, indulged in their rapidly building hatred and despair, becoming Oni that they may deliver their revenge onto the world. Given new form, these former Kami took to wanton destruction and hedonism as a way of life, holding their burning hatred of the Yema for destroying their wards, at their former Kami brethren for being cowards and weaklings unable stop the destruction of their wards and ways of life, and at mortals for failing to sufficiently appreciate the pleasures of the physical world and for being unable to stop the Yema close to their hearts as they went on to become one of the greatest evils the continent has ever known, raging at everything and everyone. As they were thus believed to have originated on Zheng-Kitar, it is also where they are most prevalent - as beings who begin existence as kami, nature spirits bound by strict divine rules known as the Laws of Golden Perfection, Oni continue to form into the present day when Kami go the route of these ancient progenitor Oni, embracing their hatred and rage to become something more corporeal. When a kami fails to live up to these laws, it is stripped of its powers and becomes a formless spirit. Most former kami simply fade away, but some are so overtaken with rage and despair that they will themselves into a physical form that mimic humanoids, becoming oni. Oni become obsessed with their new-found corporeality and revel in physical pleasures such as food, sex, and violence, and to wreak vengeance on the world for its percieved fault, doomed to never be satisfied until all of reality is yoked beneath their destructive, hedonistic rule. However, a major roadblock to the Oni's plans of universal domination quickly appeared before them - as beings that were and remain innately spiritual beings, Oni do not have a true physical body until they can "manifest" one: Meaning that while many Oni might exist at any given time on the Material Plane, many millions more exist in the formless Spirit World as spirits unable to properly form a body, existing as disembodied evil longing for sins of the flesh, harmless and invisible. It is only at certain places or under certain conditions that an Oni can properly take on a physical form - such as The Boiling Lake in The Narixian Empire, which is believed to be a natural "cradle" that allows them to form with astounding speed within its watery depths. Now, they exist as one of the greatest threats Zheng-Kitar has ever known - as beings that were once spirits tasked with protecting a material realm they now believe is undeserving of such care and concern, they exist now solely to dominate, consume, and indulge without end until the very last drop of blood is spilled, the last scrap of food is consumed, and the last creature is dominated and broken before them.
Oni are rare only because of how closely their appearances are watched for and kept in check - if such a vigil was stopped, they would surely overrun the continent in short order.
Varies wildly. Oni have been recorded as small as 2ft(0.6m) tall and, in one legendary instance, as tall as several miles/kilometers tall.
Though their physical size can vary wildly, Oni are universally humanoid beings that can appear as both male and female, appearing both lithe and heavy-set, though they are more commonly seen with fit bodies ready to indulge in slaughter and death. Their bodies process food and calories incredibly quickly, meaning that while they can get large and obese, it is extremely rare for them to do so.
Body Tint, Colouring and Marking
Oni bodies can vary wildly in color, but are rarely "normal" colors - blood red, bright green, dark blue, and similar colors are common, with dark charcoal black also quite common.
Born from Mortal SoulsKami, a very rare few exists who are born from the souls of mortals instead - rarely, a mortal creature’s soul can become a disembodied oni upon death if it was suitably vile or twisted in just the right ways to allow for them to become an Oni, or in even rarer cases, a truly evil individual becomes an Oni after it has undergone a particularly vile ritual that ends in suicide, binding its soul to the Spirit World and damning itself to an eternity as a twisted Oni. These oni born from the darkness within Mortal Souls(Either intentionally or not) are more often destined for positions of great power and strength than most others of their kind.
Beings of Unnatural MightAnother terrifying power all Oni possess is that of regeneration - as they are naturally formless spiritual beings, they retain a strong connection to the Spirit World even after taking corporeal form, and it is thanks largely to this connection that their flesh knits itself back together at a startling rate, making them nigh-unkillable unless any given oni's specific weakness(Often elemental in nature, such as fire or acid) is found and deployed against them to sever this spiritual connection and render them truly mortal long enough to kill them permanently...otherwise, simply killing them with mundane means will only delay the inevitable, as their bodies can recover even from total atomization unless this spiritual "Link" is severed.