User blog comment:IamJakuhoRaikoben/Chapter 358: Magical Barrier Particles, Review./@comment-4489126-20131101171232/@comment-4829046-20131101180924

So? END could be working independently, doing covert work for Zeref. Even Makarov kept some secrets from the other FT mages. Even if Tartarus has no knowledge of FT, there is nothing to say END who is ancient and able to survive against Igneel is the same. He is shrouded in mystery after all, as he(she?) should be, since the first mentioning of the name was very recent. This is all just speculation.