User blog comment:FairyRaven/Fairy Tail Chapter 407: All for My Demise, Review and Commentary/@comment-213.89.168.146-20141101142502/@comment-7418318-20141101212537

Maybe Zeref just thought that the seal on END couldn't be broken by anyone. But while he was sulking around, Mard Geer (or rather his subordinates) found out about Face and thus a way to awaken END after all, which Zeref still might not know about, or at least didn't back on Tenrou.