User blog comment:IamJakuhoRaikoben/Chapter 418: Challenger, Review./@comment-24546844-20150202172212/@comment-7418318-20150203000655

Tartarus were never said to be Zeref's elite force. They were just a couple of Demons who E.N.D. formed into a guild.

But even if the Tartarus Demons were the best that Zeref could create back 400 years ago, he might have improved his skills when he created E.N.D. and thus might now be able to make even stronger Demons.