A medieval world, centered upon an isolated island that is split into several city-states and major provinces. Humans tend not to war with one another here. They are plagued with a much more serious problem. That of the Yoma. Demon-like creatures with fearsome strength and agility, that feast on human flesh, shapeshifting into the forms of other humans, to kill with stealth. No human can match them on even terms, and they know this and continue to hunt.
Humanity does have weapons to fight them, however. Other humans, who are infused with the flesh and blood of the Yoma themselves, turning them into half-breed soldiers with the Yoma's strength and agility, but with the bodies of a human. Only female are capable of being useful, and these warriors, called Claymores by the human public, are created and regulated by 'The Organization'.
But, they require a fee for their service. If a town refuses to pay, it is never helped again, and often beset upon by Yoma. Claymores themselves are dangerous, as their powers require they walk a razor's edge between their humanity and the power of the Yoma within. If they exceed their limits, they risk becoming a much more lethal version of what they were made to hunt.
Yoma are now able to spill outward into the Multiverse, forcing Claymore to step up their efforts, while their strongest warrior is still at large with a death sentence on her head for breaking one of their most rigid rules.