Onikiri is a great katana that provides 74 damage, 450 delay, and Critical hit rate +5% against demons. It deals 9.87 damage per second and accumulates 115 TP per hit. It can be equipped by level 65 Samurai. It is dropped from Oni Carcass.
Demonslayer is a katana for Jack. It is a mid-powered katana with attack power of 42. It is a mission reward for Wyverns in the Woods (S-Rank on Finis difficulty) and received for fulfilling a Special Order in The Last Line of Defense (Performance Evaluation: Defend allies). It sells for 1800 gil.
Onikiri means "oni-cutter" in Japanese.
Oni is the kun'yomi rendering of the Chinese interpretation of an ogre called the "guǐ" (gu-ay). They are a kind of yōkai from Japanese folklore, variously translated as demons, devils, ogres or trolls. In a Chinese context it may refer to a ghost as well.
Though its name was generally popular for katana throughout the ages, its most famous attribution would have to be for being the nickname of the sword of Watanabe no Tsuna, a retainer of Minamoto no Yorimitsu in the late Heian Era. According to legend, it was powerful enough to cut off the arm of Ibaraki Doji, the powerful second in command of Shuten Doji, the feared demon lord of ancient Japan, and immediately halted his rampage on the city of Kyoto.