User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-24277053-20150518124050/@comment-4933386-20150518132126

I think transforming into a dragon a final stage of Dragon Slaying magic. It may explain why Zeref questioned his power and ability to fly when they encountered each other. which indicates his power is not strong enough.