Daemon Sword Medium  

The sword you are referring to is known as a "living sword," similar to Grospoliner and Gesso. However, unlike those two, this sword has an evil nature that feeds on suffering and completely controls its wielder. To break the control of this sword, one must have it deliver a fatal blow to someone they love. In the series, Mana wielded the sword before Katsumi possessed it in the later parts. Kiddy's intervention was successful in breaking the sword's control, but she survived due to her android nature.