When the Saint God was heavily injured by the Ancestral Dragon’s will, the will of the Ancestral Dragon became more of a poison that tortured the Saint God's body for tens of thousands of year. However, the Ancestral Dragon's Will was not enough to wipe out Saint God. Instead - mush like how Zhou Yuan utilizes the Dragon Resentment Poison - the Saint God was able to force out the Ancestral Dragon's Will and distort it to use as a material to make the Absolute Divine Curse Poison.
This poison was specifically created to target Yao Yao; the poison will devour her divinity and corrode her body until she will enter a self-protective dormant state. In order to expel the poison, her divinity will have to completely awaken and thus become the real 3rd God.