カースドソード
from Monster Girl Encyclopedia
Origin
Ancient structures
Former human women who became entranced by cursed swords. Generations ago, these cursed swords were created by a wicked demon lord who imbued them with malice. In those days they were fearsome weapons that would cause a human who picked one up to go mad and change into a mass murderer who would even slay his own comrades. Their nature has changed along with the transformation of the monsters. Just like the “demon realm silver” weapons used by monsters, they inflict wounds on an opponent's “spirit energy” rather than their “body", and they don't kill. Instead of feeling painful, the wounds feel hot and throb; and instead of blood, the body's “spirit energy” leaks out, only to be replaced by demonic energy from the cursed sword, changing the victim into a monster. Due to the danger they once posed, they were sealed away in various places by the current demon lord, but with the passage of time the seals have weakened and many have been unearthed by illegal diggers. At a glance, a cursed sword looks like an ordinary sword, but those who carelessly pick one up will be parasitically infested, their body and mind gradually corrupted. By having the host cut people, the cursed sword sucks up mana, growing powerful and sinister. At first, there are only minor changes to the host, such as an elevated mood and increased belligerence, but little by little the sword's influence grows, and the host will end up being compelled to cut humans with it whenever she sees them.
Monster Girl Encyclopedia
