The transformation from True God to Main God is most difficult at the stage of condensing the divine core.
Many Nine-star True Gods can easily accomplish the step of opening the Divine Sea, but ultimately collapse at the stage of condensing the divine core.
Even if the Divine Sea is stabilized successfully, if the divine core cannot be condensed, the Divine Sea is like a body without a heart, a dead thing. After some time, it will collapse and be destroyed.
As the Divine Sea is destroyed, the Dao Palace will also suffer great damage. Hao Yang suffered major damage to his Dao Palace due to failure in condensing the divine core.
For a Martial Artist, condensing the divine core is, simply put, the refining of the Laws. Using the Power of Laws embedded in the Divine Sea as the foundation, one condenses one's own divine core.
