User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-29874610-20170204151505/@comment-5303512-20170206005303

There was no mention of any Demon Seed being implanted in Natsu at the beginning though, although I will say something could have happened that if there was a demon seed, it was removed somehow, and when Zeref pierced the Book, the demon seed formed again inside Natsu when he got that Growth.

Mard only lied about END creating Tartaros, unless there is proof Zeref saying he was lying about everything else, Zeref even congratulated Mard for almost resurrecting END