User blog comment:FairyRaven/Fairy Tail Chapter 349: The Demon Doriate, Review and Commentary/@comment-33019320-20130830102126/@comment-7418318-20130830131136

With Gray getting into contact with Deliora (twice), Lullaby and being present in the Eclipse Crisis I suppose he's got quite a bit of experience with how Zeref's creations feel like.

And Hades' summoned Nemesis demons might have had a similar feeling to them too, since that spell also came from the Book of Zeref.