Demonslayer, called Onikiri in the Japanese version, 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 are a kind of yōkai from Japanese folklore, variously translated as demons, devils, ogres or trolls.