The Soul King

Originally, there were two "supreme beings" that worked together to create the three realms: Hueco Mundo, the World of the Living, and the Underworld. They created the soul cycle, souls, nature, and everything else within these realms. When their goals for the worlds differed too greatly, the two battled until the one we now know as the Soul King was victorious. When the other was killed, its power scattered and latched onto different souls as they moved through the soul cycle. The Soul King, after assigning the original Gotei to guard over his palace just in case the other would return, split his body into pieces in order to create keys for his guards.