Fairy tail chapter 251
I don't know if acnologia is really a dragon...i mean he looks a bit weird even for a dragon, he has some strange markings on its skin. I don't think that he is the black dragon, maybe he is the dragon king himself that igneel was talking about.
I personnally don't like the idea that zeref could be a dragon slayereven though it may be true. If zeref dragon slayer power is to make living things die, then what kind of power could acnologia hold? It could then litterally take life from living things, acnologia would then be too powerful. Anyway, with this creature, tenrou island is definitely going to be destroyed... unless mavis himself decides to lend a hand. I also don't think that fairy law can make such a big difference.