User blog comment:IamJakuhoRaikoben/Chapter 357: The Nine Demon Gates, Review./@comment-112.209.220.215-20131025145720

Theory about E.N.D.:

http://readms.com/r/fairy_tail/357/2117/8

Judging by the picture, E.N.D. is the skull in the bone-like cross. He may not be living/existing yet and might be communicating with its servants telepathically (through the book of E.N.D. that they possess, I presume). Since their core team is named "Nine Demon Gates", it probably has something to do:

1) Their servants following the instructions to summon E.N.D. through the book. 2) Requirement of an item needing to be obtained by the demons in Earthland (probably explains why they want a full-scale war when in the first place they can easily obliterate the world (Jackal destroying the Magic Council headquarters by himself. Does that ring a bell?) 3) (most unlikely) The defeat (like how it happened in the Oracion Seis) and or sacrifice of the Nine Demon Gates. (If this is so, then they most likely would have killed themself for master Zeref.).

Related to theory #2: Maybe the items required to summon E.N.D. are the books of the Nine Demon Gates (all of them are from the books of Zeref.) and all the books are scattered in difficult-to-find locations in Earthland (probably why they want to invade Earthland)

Related to theory #2 and #3: The sacrifice of the Nine Demon Gates using their respective books when they obtain them all.

When these "theorized" requirements are complete, they will summon E.N.D.

Again, this is just a theory. That is the power of speculation and observation.