User blog comment:FairyRaven/Fairy Tail Chapter 370: Demons Reincarnated, Review and Commentary/@comment-1359322-20140207104913/@comment-7418318-20140207111142

Regarding Jackal: In the past there were theories that the Demons can only be fully killed if the "Book of Zeref" from which they were summoned is destroyed.

I guess Jackal's return give those theories some more credence.