>>714218187That wasn't actually her.
Gwyn went and bured his power till he became a hollow to keep the age of fire going.
But before that he worked with Frampt and Gwyndolyn (Snake Twink) to manipulate Undead into gathering up the other lord souls and getting strong enough so that when the fire stated to fade again that "Chosen Undead" would be strong enough to be worthwhile fuel.
In truth sometime after Gwyn burned himself all the gods but Gwyndolyn left Andor Londo so Gwyndolyn used his illusion magic to make a fake Gwynevere to lead all the peons living the the lower areas we never go to, and to trick undead that make it that far with big boobies.
As for killing Gwyn everyone just assumed he was dead instead of continuing to exist as a hollowed out shell till you put him down.