User blog comment:Sargenttacos115/The side efx of Dragon slayer Magic/@comment-1359322-20131109143240

Hmm i think the Dragon Slayer magic became a Lost Magic because only a few Dragons are left on Earthland. Side effects, haven't seen one yet, the Acnologia one is still a mystery though, it could be because he bathed in too much Dragon blood, and continuous usage of the Dragon Slayer magic made him evolved. There's also a theory of Zeref helping Acnologia in his transformation.