True Will

Zatiel was in a meditative position inside the black dimension, and his face was perfectly calm.

One's soul is constituted by the Primordial Essence formed by the fusion of a piece of the parents' Primordial Essence and soul force belonging to The Eternal River of the Afterlife. Only the core of your soul can be considered truly yours.

Once an individual reached Peak Rank 3 and comprehended a law to the Potential level, his soul would undergo a qualitative change. Under the guidance of the Universe Will, his Primordial Essence would begin to consume the soul force that was granted to you by The Eternal River of the Afterlife. 

When this process is done, your Primordial Essence, the part that is the core of your existence, would have grown powerful enough to be solidified and form the True Soul. This massive change is the reason why Rank 4 life forms are so powerful and can have access to devastating powers like Law Avatars.