Although this sword is powerful in the hands of a Blackguard, the blade chooses who is worthy to wield it. Many who have wielded the blade and were not chosen were paralyzed completely.
The effect of the paralyzation can be removed either by the chosen wielder of the blade or if the sword requires your assistance in the aid of finding its true master, however it will not grant any magical properties to that wielder.
After an unknown period of time, the wielder's soul will eventually be consumed by the blade's dark energy, causing the blade to act as a unique type of phylactery.
The one who forged the blade is unknown even to this day, but there are telltale signs that the creator did it in the presence of a dark god.