The Soul King. A being of untold power that held all the worlds together. Without his presence, the world of the living, the soul society, hell, hueco mundo, and the dangai would fall apart and be destroyed. He kept a very small portion of his power in a palace guarded by the zero squad, taking the form of a limbless body in a crystal.
However despite his power, the soul king was worried for his worlds because of the existence of one being, someone so unnatural, someone with enough power that he threatened to destroy the worlds if he was ever allowed to reach his full potential.
This someone, was a carrot topped human by the name of Ichigo Kurosaki. A being whose soul had the power of a soul reaper, a hollow, and a quincy. A being who surpassed even Sosuke Aizen, who got closer to being as powerful as the soul king with each passing second due to the hogyoku. However, he could only go so far with the help of the hogyoku.
Luckily for the Soul King, there were many ways he could deal with the human. He could simply erase him from existence, he wasn't at a level where that would be impossible. He could also take away his powers and leave him to leave the life of a normal human.
Yet the king wasn't without mercy. And he was going to need some form of entertainment for the next few years.
It was then that he made his decision. Too make him a child again. To leave him with the ability to regain his powers and reach his full potential.
To take away his memory and leave him to flail in a world made by lesser gods.