Made To Be Weapons
Sword-lore is an exotic art even in the land of the living, but when one has passed into the next life, it becomes even more esoteric, for in that uncanny and strictly-policed land, true swords have strange powers. True swords have strange names. True swords have souls.
Of course, all souls have to come from somewhere, and the source of a zanpaktou's empowering soul is a question much-debated in the musty scholar's halls of Soul Society. The prevailing theory of the day is that a zanpaktou is part of the user from birth to death, and it awakens within the prospective Shinigami as they pour part of themselves into the blank slate of their weapon. The zanpaktou spirit is not an entirely separate entity, the theory states, but rather a budding off from the wielder's own soul, taking a new form to aid them in combat.
This theory is neat, concise, and explains many of the idiosyncrasies of zanpaktou. It is also totally wrong in almost every respect.
Zanpaktou spirits are souls, but not of this world. They come from somewhere else, from a wheel of reincarnation quite removed from our own. They live, fight and die in their world, and are borne into Soul Society, altered and reforged by the space between worlds into weapons, to be wielded by the protectors of the afterlife itself.
There are many from that other world who would be quite pleased by this fate. Shinobi are made to be weapons, after all.
Just a silly idea I had a while back and finally decided to put down. There'll be a short per chapter, and each one will likely be rather shorter than this. I'll upload them as I write them.
-Revlid