Based on Asa's description, the Supreme Will should be a corrupted and fallen ancient god.

Killing a fallen ancient god to end the black mist polluting the world seems unbelievable.

The most crucial point is that there is no necessary connection between purifying the High Spirit World and killing a fallen ancient god.

In the High Spirit World, besides ancient gods, there are also the Old Ones, corrupted kings, black mist great demons, and other terrifying entities. Only by eliminating all of these can we gain control of the High Spirit World and achieve ultimate victory.