A/N: This is my theory about zanpakuto, hinted at in "Life" and inspired by the Dune series. Pure speculation on my part with no actual canonic basis as far as I can tell. In other news, if any of you are Ouran fans I've posted a Kyouya-centric oneshot (this is an awful lot of fanfic for exam season methinks ...).
WARNING: Seriously dark. Seriously. Inspired by discussion of royal tombs, that should give you a hint.
The birth of a zanpakuto is unlike that of a living soul. A zanpakuto is not born, but rather created, not consciously, but with a single, constant purpose. To fulfill our purpose we are granted the knowledge and memory of all those who have been created before us.
I carry with me the lives of a hundred thousand zanpakuto and their hundred thousand Shinigami. I have thought their thoughts, lived their lives … and died their deaths. A hundred thousand lives as unique as snowflakes and a hundred thousand deaths each the same.
Every zanpakuto exists in a separated corner of its master's mind, an isolated inner world, and it is to this world that every Shinigami goes as they tumble over the edge of death. This is the way every life I remember has ended - with ground and sky pressing together, closing like the walls of a tomb.
And after that … not even a hundred thousand minds can see. Perhaps we stay forever between those opaque walls. Perhaps the tomb keeps closing until it folds in on itself and disappears. Perhaps only I will disappear, while you are re-born.
I know your story, beginning to end. You are Shinigami, as they were; we may live together for a thousand years but one day you will reach into my world for the last time and we will fall, you and I, beyond the cold horizon.
That is the position of the zanpakuto, standing as gatekeeper between your life and death. Tombkeepers watching as you enter and exit, again and again, unknowing. Hating myself for the day I will let you in, and lock the door after me, and you will not be ready to stay.
I will make sure you die happy.