In the beginning, there were three beings of power. Light, Freedom, and Darkness. These beings took the form of Gods. The Soul King, Humans, and Lucifer. As the time grew so did their power. Eventually both the Soul King and Lucifer became to powerful for their own body to hold it. They, in turn, split their power into other beings. The Soul King created Shinigami and Zanpaktou, while Lucifer created Hollow and Sin. And, Amongst this splitting, the Humans created Intelligence.
However, this created problems, large problems in fact.The Shinigami and Hollow did not even begin to get along with the humans. It came to past that no one could get along between the races. The hollows sought intellectual from the humans, and the Shinigami sought more power from the hollow. They began to fight and it so forth came that Shinigami were the protectors of humans, and hollows were the sworn enemy of all. Because of this the Soul King created Soul Society and placed all the Shinigami into it. Lucifer created Heuco Mundo and placed all the hollows into it.
Once again, turmoil began to come over the world. Hollows began to escape and disobey their master. They lusted so much for the intelligence of humans that they started to rip the dimensions and feed on them. Pissed, that the hollow broke their deal, the Shinigami came out of their dimension and began to attack them again. However, they did this for a different reason now. It was to protect the humans. To protect the humans, the Shinigami began to form contracts with the Zanpaktou. They saw that this power was immense, and in turn they began to enslave the Zanpaktou and force them into weapons.
The Soul King became angry at this, and to stop the cruelty to his children he sealed zanpaktou into the very souls of Shinigami. Because of the protection that was now given to the humans, the humans decided to make a contract with the Soul King. They gave up their immortality and in turn gained souls and make the Soul King more powerful. But he was not happy. His children were still fighting and dieing in wars against the hollow. He decided to make a deal with them. He gave them parts of his power and the human intelligence, and in turn they gave some of their power to the Shinigami.
Those who shared this power were named Arancar and Vaizards. But the deal was filled with sin. The power was not split even. Those Shinigami that had taken in the hollow's power began to become hollow themselves. Eventually shinigami shunned the hollow's part of the deal and the arancar and vaizards died down.
Everything was going fine, the balance between the powers was nearly perfect. However, the power of individuals began to grow, and so came individuality. To prevent things from getting out of control, the Soul King gave power to special individuals in the form of God-like zanpaktou. These people are the chosen users of the Soul King.
(Others can continue on this but this was just to get down the idea for now)