Just as Humans and Meisters are susceptible to the afflictions of Madness after the consumption of a pure human soul, Demon Weapons are also capable of descending down the path of the Kishin and losing themselves to their insecurities, becoming refered to as a Dark Weapon instead.
Dark Weapons are born of the particular individual's fears and althougth these vary greatly dependent largely on the past events of the specific weapon, it is the succumbing to these vices which begins the descent towards becoming a Kishin. Eventually the weapon's desperation to be rid of these fears, develops into an insatiable craving for power that inevitably drives them to consuming the soul of a good human. The ingestion of such a pure soul corrupts the weapon in question, meaning that even with the substantial boost in power afforded by the act, they find that neither their fears or hunger are quenched in the slightest. This begins a vicious cycle in the consumption of a significant amount of human souls, until they either achieve the status of a Kishin or are hunted down and killed at the hands of Shibusen.
ABILITIES
Dark Weapons retain all the abilties afforded to them due to their demon weapon heritage but gain effects reflecting the corrupted nature they now possess. The most notable of these is a soul that will eventually absorb the soul belonging to anyone who wields them.