User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-24277053-20150518124050/@comment-9595671-20150518131259

For the Acnologia thing, we once thought Zeref made him a Dragon but the Tartarus arc showed that was his own Dragon Slayer magic. Its possible Zeref did do something to him though but we don't know what he did.