A sword with an evil spirit inside.
The Hina Blade was a cursed katana containing the spirit of an ancient enemy of the Shinmei-ryu swordmen. The sword could possess its wielder and sap them of their soul or grant its user greater strength and power at the cost of their mind. The Hina Blade was responsible for causing a great deal of destruction to the city of Kyoto centuries earlier and it took the entire Shinmei-ryu clan to slay the enemy wielding the sword and seal its effects. The sword was eventually passed down to the Urashima family as an heirloom.
In the present, Keitaro discovered the sword and gave it to Motoko as a replacement for her sword that was broken by Tsuruko, unaware of its evil nature. While Motoko could sense its nature and refused to wield it, Naru would wind up being possessed by the sword and going on a rampage with it. Motoko was able to seal the sword before it could cause too much havoc and would make use of the Hina Sword as her replacement sword for battle.