Oni and Tengu populate an entire country's worth of what essentially is the Japan of the world.
They are extremely war-devoted, and take extreme joy in crushing their foes and displaying the dead's skulls on their bodies.
Oni will quickly smash foes up and close, while tengu will take their time slowly degrading the foe from afar on their raiju mounts.
Due to their constant bloodlust, oni are in a constant civil war between themselves and other oni, as their own worst enemies are themselves. The greatest fight to them is their own kin, as man cannot provide due to weak bodies and relying on armor to win.
Tengus are less prone to civil war, but always get bossed around by their oni bosses. They COULD feasibly overthrow them with enough willpower, but their spineless nature and the fact that the oni are the ones that supply them in general keeps them from betraying.
Onis and Tengus come in three colors, red, green, and blue. Red ones are plagued by rage and are most likely to get into war, blue ones are paranoid and try and fight anything that's even close to their size, and green ones are nomads, preferring to just go to villages to fuck them up.
Once every thousand years, it is said that a black oni, often called the "Osoroshi" by other Oni will be born, and unite all Oni to conquer the world. There is said to be a new one, going by the name of "Skulltaker", which is currently forming a small army of red, blue, and green oni and tengu.