User blog comment:FairyRaven/Fairy Tail Chapter 372: Breach, Review and Commentary/@comment-24039250-20140221160151/@comment-7418318-20140222095838

It's possible that this curse might bring stuff from Natsu's subconscious up to the surface or somesuch. With him currently fighting Demons from the Book of Zeref, him halluzinating about Zeref would make sense.

If the curse is just intended to distract/disable the target for a short time to allow the Demon to walk up to it and whack it over the head (or stab it or whatever), they wouldn't care about what the victim halluzinates about.