Oni across Uikku and much of the lands that once belonged to the Takani Empire have exploded in numbers in recent years. A string of back to back wars, famines and plagues has seen the lands ravaged and thousands of ancestral lines end. The ancestral spirits that guarded that watched over those lines have thus lost their way, without purpose and fuelled only by anger and resentment towards the surviving lines and spirits, especially those responsible for ending their own lines.
Thus another war has swept over Uikku as Oni begin manifesting within the physical realm, terrorising villages and townships in a genocidal rage, in turn creating more Oni in their wake. Eventually their enemies will become themselves in a demonic civil war that will engulf the entire spirit world and spill into the physical realm. So the Spirit Samurai and the few remaining benevolent spirits must do what they can to contain this threat.
The Onslaughters make up grunt-like Oni that are seen as disposable fodder and are sent into the fray against mundane swordsmen and humans. They possess no outwardly powerful magics apart from their ability of regeneration in which they can quickly heal mortal wounds and even regrow severed limbs, provided they have a human body nearby to feed on. But witnessing the grotesque matter of an Oni feasting on a corpse curdles the stomach of even the most hardened soldiers.