User blog comment:Fusedtwilight/Nature of the Soul King/@comment-24769684-20151225044047

Where was it stated that the Shinigami created the Soul King? The other way around would make more sense.

Anyways, there's still a lot of things to be revealed about him, but here's my theory: in the early days of the universe, the Soul King was out and about. In addition to keeping the worlds in balance, he had nigh omnipotence and omniscience to boot. With his right hand, he could make places decay, but with his left hand, he could shake things up by making them evolve. And his heart, well his heart could work miracles. Nothing could bring him down, as he turned any damage he took into strength.

He asexually gave birth to Yhwach, a son in his father's image. Yhwach hated his father, who was never there for him, and decided to use the universe to build his own Empire. Yhwach waged war on the Soul King and Soul Society, but was defeated by Yamamoto. However, Yamamoto failed to kill him out of fear of the Soul King's wrath.

After Yhwach's defeat, the Soul Society realized that the Soul King was too powerful to walk among them and reproduce equally powerful beings. With the Soul King's consent, they sealed him away, removing his power while keeping him alive in order to preserve balance. They also sealed away his three main sources of power, two of which were eventually stolen by the Quincy.

Kinda went a little far here, but that's what I think happened