These jet-black longswords are magical weapons, made in ancient times by mysterious and long gone caste of the Worshippers of the Eternal Darkness (maybe they worshiped Nyogtha). There are at least six Black Swords, but its current location remains unknown.
Each Void Sword is a +1 weapon, but if it’s used against any sapient opponent, treat it as +3 sword. It deals +2 damage against sentient beings and any victim of 1 HD or less must make an successful save vs death or is killed outright. However, use of this weapon affects its bearer – as time passes he (or she) becomes increasingly cold and inhuman, as if the chill of Eternal Darkness passed to the holder of the Void Sword. Every time a sentient being will be killed with this weapon, roll d6. On a score of 1 reduce wielder’s Charisma score by one point. This loss is permanent.