Wang Yang did not stop the Demon Ancestor from leaving, of course, he didn't have the means to do so. The Demon Ancestor destroying one of his avatars directly indicated that he had abandoned this Square World!
However, even though the Demon Ancestor retreated in defeat, Wang Yang knew this was far from over. It was not just because of the Demon Ancestor's character, which would not yield or give up, but also because Wang Yang believed the Demon Ancestor's words were not mere exaggerations.
It is quite possible that the Demon Clan has already infiltrated countless Domain Planes, and just as the Demon Ancestor said, Wang Yang cannot go to every world to prevent the Demon Clan's invasion.
Wang Yang understood this principle long ago, but currently, there was nothing that could be done, as the forces of the Demon Clan are intricately intertwined, and their numbers are unimaginably vast.
