User blog comment:CNBA3/Tartarus' Army!/@comment-46.198.201.133-20131105182205/@comment-24066070-20131105204704

Unless Zeref went on a transformation spree in the past. He turned Acnologia from human into dragon, so perhaps he turned some humans into demons as well.

Although it would be better if they have always been demons, otherwise FT will turn them human again with their nakama speeches.