The evolution from mortal flesh to golden muscles and jade bones is precisely the beginning of "Stone to Gold", which is essentially Body power.
In ancient legends, Immortals often possess the art of "Stone to Gold".
This legend, in fact, already hints at the mysteries above Returning to Void.
The Returning to Void Realm is merely the starting point of Body power, while the Unification Realm is the perfect fusion of Primordial Spirit, Mana, and flesh.
From then on, those in the Unification Realm completely shed their mortal flesh, not only controlling union at will but also creating avatars, roaming the heavens freely.
As for the Tribulation Crossing Realm, it probably signifies the true body, primordial spirit, and mana all beginning to embody the Law.
Though the above is still just speculation and hasn't been truly verified, Zhao Sheng is ninety percent sure his speculation is correct.
