User blog comment:Omega natsu2/Chapter 394: Juvia vs. Keith, Predictions./@comment-1359322-20140725081250/@comment-25244540-20140725082123

So Gray's now an Ice Devil Slayer? That's wicked, can't tell if it will be permanent or not. I hope it is because since Zeref seemed to have been the reason Acnologia became a dragon, I feel Acnologia is a Dragon/Aetherius Demon hybrid and that could be useful against him.

And I think we learned something about demons that's a fatal weakness for them, they're overconfident. Especially with humans to the degree Keith released he fell for Juvia's trap because he had been too careless.