Seth's power was already tyrannical by nature and when it was being handled by the Great Corruptor, not only he took the concept of tyranny to another level, but he was also so damn powerful, that it got to the point where nothing could really resist him.
The Illusive Ethereal Goddess could not find a single spec of strength to resist the power that was consuming her, much less fight back against it. Corruption started a chain of evolutionary reaction inside of her and nothing could stop it.
It only took a few seconds for the entirety of her body to change from purple to crimson, but for the goddess, time kind of passed by in a slow manner. Painlessly, she failed her evolution and her powers were thoroughly corrupted, forever changing to be part of something greater.
Defeated... The Illusive Ethereal Goddess had finally been defeated.