Two was once One

A long time ago before the Soul Society and Huco Mundo came to pass. There was a dimension that held "demons". Creatures of a god like nature. They were all very similar and moved as a whole. Till one day a revolution of sorts occurred. They fought over how to deal with a new problem that confronted them, humans. Some thought they were only good for devouring and toying with. While others wanted to help and protect the slowly developing humans. Which after a bloody and gruesome war the "demons" split and went there separate way. Then because of the "demons" no longer being as one they later changed into what we now know as Shinigami's and Hollow's. The Shinigami's souls turned into Zanpakuto and they kept there Kido, although they now have to recite incantations unlike when there in there "demon" form. The hollows kept their powers through the form of there Masks and there body's transformed when they fight.

Soon after the split a small amount of the "Pure Demons" stayed together and constructed special dimensions for the Shinigami and hollows to suit their needs. Once completed and after the Shinigami and Hollows had settled. They asked the "Pure Demons" for a way to keep in contact. So the Soul Key was constructed by the "Pure Demons". The Shinigami and hollows became the holders of the key while the "Pure Demons" left to their dimension to resume their task of keeping the dimensions intact. Soon after the "Pure Demons" were given the title of Soul King as a show of respect to them as a whole. Because of this peace was brought the dimensions for many millennia.

So with no need of the "Pure Demons" they became a myth, till one day they were no longer remembered at all. The Soul King though became an it, like an object instead of what it truly was. The "Pure Demons" themselves never forgot there end of the bargain. Over time the bargain became harder and harder to keep. For you see the "Pure Demons" began to long for their existence to end one by one. The only way though for a "Pure Demon" to die is for it to be willingly be killed and eaten by another "Pure Demon" or else they would regenerate. Then the one devoured would be absorbed into the other "Pure Demon". Soon the numbers dwindled to 100, 50, 30, 10 and till finally there was only one "Pure Demon" left. Since there was no one else to devour her and no one else to hold the dimensions together. She was left alone to forever uphold the bargain. Out of loneliness and boredom the "Pure Demon" began to watch the dimensions and there never ending liveliness. Eventually this made the "Pure Demon" even lonelier, deciding to end her solitude she decided to go the land of the living and begin a new story of her own.