User blog comment:LegerPrime/Acnologia the Ethernano Dragon Slayer/@comment-1776824-20150612174035

Acnologia was confirmed by the other dragons to have transformed of his own accord due to overuse of his magic (most likely a side effect of Dragon Force, yes), and the bit from Arcadios about Zeref having a hand was just speculation on his part. His specific type of Slayer Magic was said to have involved soul-reaping. No idea what the hell kind of element a "soul" is, but that was the power Igneel said his magic has.