Blade of Salvation is one of the magic swords described in Monster Girl Encyclopedia World Guide - Side II.
Originally, it was just an executioner's sword and not magical, despite its elaborate decoration. Since the fall of Lescatie, however, it has been changed into a mamono realm sword with ability to detect hostility toward mamono or their husbands in the opponent's heart, and capable of moving its wielder’s body to strike such a target. A woman "beheaded" by a blade of salvation would become a dullahan, while a man would fall into a near-death state, as if the man had eaten an undead grey lamentation mushroom, waiting to be rescued by a mamono.
The blade of salvation has been multiplied since the fall of Lescatie, with each dullahan monsterized by it carrying her own blade of salvation. And even its old victims before the fall are now raised as dullahans equipped with the blades.
Guide Book Entry
"The Blade of Salvation" was the weapon used to execute sinners in Lescatie during the reign of the Order. For a religion that classifies all monsters as evil and seeks to eradicate them all, interacting with a monster and finding a rapport with it is first and foremost a grave crime. In the religious state of Lescatie, under the careful rule of the Church, all people who were considered to be close to monsters were declared traitors and imprisoned, and then they were beheaded with this blade, under the guise of "saving a soul trapped by the monsters." This "Salvarision Blade of Salvation" was an ordinary sword for execution, but it was richly decorated, like a ritual blade or a treasure sword, and gave the impression of a sword wielded by a hero. Perhaps the church wanted to emphasize more strongly that this sword was a "sacred blade for the salvation of the soul."
This blade of salvation with a bloody past is still used to "save" people in the present mamono realm of Lescatie. But now the "relief" given by this blade has a very different meaning from the "execution" of the inhabitants in the former Lescatie. Now, because of the grief of the people and the compassion of the merciful monsters for the people who were executed by it, this sword has become magical. By becoming a magical blade, it gained the power to "save sinners," but the sin from which it is supposed to save man was changed to its exact opposite. In other words, for this magical sword, a sinner is one who "feels hostility and hatred toward monsters and their husbands." Having found such a sinner, regardless of the user's will, the sword moves the owner's body to him and tries to decapitate the sinner. No trick will work against someone who uses this Blade of Salvation. If a soldier of the Order feels hostility towards monsters, no matter how well camouflaged, no surprise attack will reach the owner of this sword.
If a woman's head is blown off by this sword, she will turn into a headless monster "dullahan" and become the new executioner for this sword. The man, however, will not be decapitated by this sword and not even a cut will remain on his neck, instead he will fall into a state of half-death, as if he had eaten the "undead grey lamentation mushroom." Eventually, he will be "rescued" from the depths of pseudo-death by the hands of monsters. More often than not, the necessary help will be provided by the undead, in particular by the Dullahan that happens to be nearby. In addition, those who once fell victim to the "blade of salvation" returned to life as Dullahans with the mamono mana appropriate to this sword. They now roam Lescatie and the "blade of salvation" that was used to execute them appears from time to time in their hands. The blade itself may not be a threat to those who are not hostile to the monsters, but to those who experience this sin, on the contrary, it has a powerful effect. The story is recorded of how this sword once accidentally fell into the hands of a soldier serving in the Order's invasion expeditionary corps, using it to slice through their comrades one by one, turning them into Dullahans and incubi until the entire corps turned into monsters.[1]