User blog comment:IamJakuhoRaikoben/Chapter 497: The Winter Mage, Review./@comment-24951162-20160810202218/@comment-5303512-20160811134104

Zeref literally said that Mard Geer only made up the fact that E.N.D. created Tartarus. Nothing else about Curses, nothing about revival, nothing about E.N.D., just lied about E.N.D. being the leader, that's pretty much it.

I doubt that Mashima would change his plans right on the spot since he wanted Natsu to be a demon.

Besides, the way that it was sounded, Igneel could have killed Natsu but didn't because he loved him too much, making it sound like it was in the world's best interest to kill him.

And with Curses, there was Bradman as well, someone who existed outside of Tartarus and Mard Geer didn't even know about. So Origins of Curses cannot be determined yet.

Also, Zeref confirmed that as he was congratulating Mard Geer, he was close to reviving E.N.D., and even said to Natsu "Who would reach him first, Natsu or E.N.D." And after all of their interactions together, I have no reason for Zeref to lie to Natsu even once, he was vague and confusing some times yes, but not lying.