I'm actually leaning towards White Magic's darker aspect making use of vampirism. Amdapor was brought down because of the extreme pride of their white mages, and we know that the elements ban widespread teaching of white magic because of how dangerous it is. Why? Because conjury (and subsequently white magic) drains the life force of the environment, people, or the user in order to work--Amdapor City's ruins are filled with poisons, rot, mold and all sort of unhealthy things. Their mages might've drained the land bone dry, and the elements got pissed and now only the padjali and the WoL use it.

But what if teaching of white magic survived when the Gelmorrans fled underground to escape the floods? There lies an avenue to bring out the "bad" aspect of white magic, through those white mages who continue to practice this kind of "vampirism" in secret. What if the duskwights that didn't go to Grid carry on the practice in secret? What if when the founding fathers of Ishgard went to follow Halone north, their people brought white magic with them?