User blog comment:FairyRaven/Fairy Tail Chapter 428: When We Take Different Paths, Review and Commentary/@comment-7418318-20150419081221/@comment-7418318-20150419132616

I'm not sure, I'd have to check. But it doesn't really matter all that much anyway if it was Zeref of E.N.D. who did it.

Though if it was E.N.D. itself that invented curses rather than Zeref, then the question is whether E.N.D. would have been immune to Face if it had already been out of it's book during Face's activation.

If I had to make a guess I'd say Zeref created E.N.D. as the prototype of Curses, but before he gave it a definite name or created more, his personality changed. Later E.N.D. could have analysed it's own body and used this knowledge to "create" Curses and the Demons who use them.