User blog comment:FairyRaven/Fairy Tail Chapter 365: Fairy in the Jail!, Review and Commentary/@comment-5598380-20131230042329/@comment-1359322-20131230124208

We can't really compare the Satan Soul and the Demons from the Book of Zeref though. Mira's Satan Soul probably came from natural born Demons like the ones in Galuna. The ones in Tartaros are from Zeref.