User blog comment:Omega natsu2/Chapter 377: The Sky Dragon's Imperial Wrath, Predictions./@comment-24467372-20140321225214/@comment-7418318-20140323105248

As we saw from him trying to capture Deliora (or rather sending Ultear to do it for him) it's clear that during his time on the council he was researching Zeref's creations. Thus I wouldn't be surprised if he knew about it and knew a lot of other things about the Demons too, which nobody else outside of Tartaros knows (save for Zeref himself obviously).