User blog comment:FairyRaven/Fairy Tail Chapter 365: Fairy in the Jail!, Review and Commentary/@comment-5598380-20131230042329/@comment-1359322-20131230125359

But seeing how Hades was able to create Demons with the use of his left eye, i think the Demons from the Book of Zeref are created by Zeref himself. Well there's also the possibility that the Tartaros Demons are created by other people through the Book of Zeref not Zeref himself.