User blog comment:IamJakuhoRaikoben/Chapter 411: Quid Pro Quo, Review./@comment-11721917-20141201142357/@comment-7418318-20141201201950

If Mard really thought he could wipe out Zeref with Memento Mori, he wouldn't need to go to all this length with Face to revive E.N.D.

Maybe Memento Mori was meant to be part of a combo together with E.N.D. and failed because whatever the Master Demon would do wasn't there against the Fairy Tail Mages.