Aibo had just learned a huge secret.
He had never expected the Demon he had casually captured to yield such an unexpected gain.
One of the Balam Alliance's three major allied guilds, one of the legendary strongest Dark Guilds, one of the most mysterious and evil final villains—[Tartaros].
It was actually a guild composed of Demons created by Zeref. Every member's true body was a Demon from the Book of Zeref, hence their nickname, "Zeref's Bookshelf."
The Demons had originally been created to kill Zeref. But after Zeref discovered that his creations simply could not kill him, he abandoned the entire "bookshelf" and left them to fend for themselves.
The reason Zeref could not be killed was that he had received a god's curse, granting him an undying, indestructible power. As long as magic still existed in the world, he would not die.
[Ankhseram God's Curse]
Aibo had found related records in ancient texts. Those afflicted by the curse would neither age nor die. The more they understood the preciousness of life, the more they loved others, the more they would take the lives of others.
Zeref hated this curse, and he likewise hated his immortal self. That was why he hoped the Demons he created could kill him—or so Lullaby claimed.
The true bodies of the Demons created by the Book of Zeref were all stored within "Zeref's Bookshelf," but not every Demon remained in Tartaros. Demons had lives of their own, and those with differing ideals often chose to act alone.
Even after being abandoned by Zeref, some Demons had not forgotten the purpose for which they were created. Though they could not kill Zeref, they still struggled toward that goal.
Their method of striving was... spreading fear throughout the world.
Demons created by the Book of Zeref