Dark Weapons

In theory, there is little difference between a weapon and it’s dark counterpart; in reality, they are nearly opposite ends of the spectrum. Where standard weapons advance in their abilities by consuming corrupted souls, still following the “99 + 1” rule, a dark weapon adds pure souls to their count. These souls, uncorrupted as they are, offer a higher tier of power, bringing with them hints of madness. As a result, a weapon and a dark weapon could possess the exact same soul count, but the actual strength of the dark weapon would be higher --- as would their level of insanity. Due to their more evil nature, Dark weapons are less likely to be wielded by most meisters, and thus are either paired with a kishin egg or just generally evil-aligned meisters; otherwise, they are at least partially autonomous.
There are a few things of note regarding Dark Weapons, aside from their previously stated mechanics. To begin with, the second anything, including a Weapon/Dark Weapon, consumes even a single innocent soul, they become part of the sub-category/class of Kishin Eggs. As such, any partnership or alliance of any kind with them is a risky one, and will almost inevitably end in at least one of the members of said alliance being consumed by the other. Dark Weapons are no exception, and being able to change forms more often than not makes them more powerful and thus more likely to be the one to walk away from the endeavor, in the end. Purging a Dark Weapon only removes soul count, not madness or corruption unless the purged weapon wills such. It does, however, weaken the madness.