The Adversaries
The Adversaries are a linguistic species. The Adversaries have many languages and call themselves many things, none of which mean "Adversary." "Adversary" is a English translation of a theme common to exonyms referring to them.
They are of a size to be able to control a sustainable campfire, so comparable to a human. They communicate with speech and make and use tools. They are very clever. They have culture - both their cultures and languages vary widely. They tend to live in families of related individuals, and to favour kin. They are usually observed to be omnivorous.
All Adversaries have a drive to work to see themselves and present themselves as good, and a majority try and be good by their own lights. These tendencies serve to make them highly effective and coordinated social animals, which makes them very dangerous in groups. They can work very well together.
However, they have developed a unique evolutionary adaptation to allow them to square the behaviours that drive trust and cooperation in a social setting with the ability to be breathtakingly self-serving and savagely cruel, especially to members of out groups, and to ravage the natural world with casual indifference.
The Adversaries are divided into a dizzying array of intersecting social groups, the most basic being the family or the village. These groups are held together their weak grasp on reality and tendency to believe in a shared, made-up reality, instead. Fealty to these counterfactual beliefs, frequently expressed through rituals and sacrifices and as well more subtly in destructive or obstruent behaviours that make sense only in light of these beliefs serve as their chief unifying glue as well as part of the fabric that divides in groups and out groups
The tendency to live an alternative reality of their own making would be maladaptive, you would think, and it actually is to a real extent, but the ability to square real loyalty and social cohesion with objectively sociopathic behaviours gives them a unique advantage, especially in the short term, which is about the only term they really care about.
Manipulating these belief systems can be used by individuals to both drive united action and promote division, based on their goals.
The Adversaries show a lot of variation, and there are two types of individuals who crop up at random in every community who are of note. There are those with an unusually firm grip on reality who are basically socially disabled, but are also very key to the Adversaries' ability to materially advance beyond their current dreamworlds. That sounds good, but they serve to make the Adversaries more dangerous over time, despite themselves having a precarious survival rate. Then there are the legitimate sociopaths who lack the instinct to try and be good by their own lights, who are completely free to cynically manipulate. Such individuals usually naturally gravitate to positions of power in their societies.
Adversaries live in morbid fear of attack by other groups of Adversaries, and live by the principle that a good offence is a good defense. Intercommunal conflict is endemic absolutely anywhere you find a big enough Adversary population to form more than one community.
The Adversaries are not found everywhere on this world - thank heavens. But they have a significant enough presence to be known to many other species. Although they are not always perceived as terrible and don't always act in a terrible way, they are often loathed and feared, or at least perceived as a source of trouble. At minimum they are major pests.
The Adversaries have a lot of stories to explain their origins, and other aspects of their world. A popular theme is the idea that they came to this world from elsewhere, although there are also many stories about being made by the gods in this world. The truth of their origins has not been established.