User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-24102956-20150112215027/@comment-9595671-20150115011235

END is Zeref's strongest demon and we still haven'y seen him in action, and while we know the Tartarus members were meant to be able to fight against armies here's the one thing that was never stated.

We never learned how strong they are by Zeref's standards, for all we know, Tartarus was made from his weaker demons.