User blog comment:IamJakuhoRaikoben/Chapter 476: Bid Farewell to the Mage Girls, Review./@comment-25111584-20160307210952/@comment-1359322-20160309094125

And it turns out that Zeref was just using the Spriggan 12 to power-up Natsu where their magic, skills, power etc all transfer to Natsu upon defeat.