User blog comment:Omega natsu2/Chapter 373: Let Live Or Let Die, Predictions./@comment-7418318-20140228113703

So I guess it was a thought projection of Zeref.

There was the same kind of distortions we've seen on TPs in the past right before he vanished again and from Jellal/Siegrain we know they can be made in a way that allows them to have (or simulate) a physical body and cast magic, so that would explain how Natsu's sword was disintegrated where it touched the apparition.

On the other hand, he really might have teleported there in person.

But with the visual distortions being similar to TPs, I'm tending more in that direction.