Weapon (dai-katana), artifact (requires attunement)
Omen: When the sword is first drawn by a creature it kills that creature outright. This only happens once and if the creature somehow returns from the dead, they automatically attune the sword and can then wield it normally
You gain a +1 bonus to attack and damage rolls made with this greatsword. Fushigiri glows with red light, shedding bright light for 20 feet and dim light for an extra 20 feet in the presence of immortal creatures. Once per day, the wielder can cause the sword to flash a baleful, blinding red light. Immortal creatures within 30 feet must succeed on a DC 15 Constitution saving throw or be blinded for 1 round.
Against immortal creatures it is a +3 magical weapon and if used to deliver the killing blow to such a creature it permanently kills them.
What is an immortal creature? Anything that doesn’t die permanently when killed (demons, devils, vampires and so on), but specifically, for this campaign, it is oni.