Kihaku

Kihaku is an ancient sword said to have been created by the Shinigami itself. Its a Cursed Sword which binds itself to your soul one you wield it. Whats good about it is that it wont let just anybody wield it, they have to have a great feeling of pain in them. To those who are able to wield it, their name appears on the side of the sword in Kanji and they are able to wield it like any other sword. To those who aren't worthy of wielding it which is about nearly everybody, the sword cannot be lifted, it is extremely heavy to them not even able to budge it, not even a sage or jinchuriki can. The sword is cursed, it slowly influences you towards evil, where you can go insane and that is the downside of this weapon. It is unbreakable, nothing can destroy it or seal it, and it can only have 1 user at a time until their death.