The Voice of the World made a plan. It wanted to break free from Millmeria, no matter what it took. So, it used its connection to The Hero through the Blessing of the World and infiltrated The Hero's mind and body.
Since it was a skill the world stuck onto The Hero to help him in his journey, the connection wasn't perfect, and the Voice had to bide its time for when that connection grew stronger or when The Hero grew weaker.
The perfect time came.
Not only was The Hero weak after defeating The Calamity. His use of world power through the Blessing of the World had also strengthened the skill's connection with his soul and existence.
In the original story, the Voice had guided The Hero rather than controlling him.
