User blog comment:Omega natsu2/Chapter 413: The Book of E.N.D., Predictions./@comment-162.237.73.201-20141217033512

Consider: Upon meeting Natsu, Zeref indicated that he knew who he was and expected Natsu to kill him, based on him being the son of Igneel. Natsu wasn't strong enough to kill him at the time. Keep in mind that Zeref knew who Natsu was on sight, Natsu didn't indicate that he was Igneel's son.

Igneel instructed Natsu to get the book of END to prevent his release.

Grey's dad gave him ice demon magic so he could kill END, specifically because END is a fire demon and would be weakest against ice magic.

END is the only Zeref demon who is still sealed, and supposedly the only one with the power to kill Zeref.

I suggest that Igneel (and perhaps Mavis) convinced Zeref that Igneel could raise Natsu to be powerful enough to kill Zeref - instead of END. Zeref sealed END as part of this agreement and waited for Natsu to arrive; waiting on Tenrou Island, presumably due to its connection to Mavis. We don't know when END was sealed, only that Igneel fought him 400 years ago, so there is no conflict with the chronology.

It appears that END is more dangerous than Zeref. As Igneel isn't worried about defeating Zeref, only about preventing the return of END. From this, I would guess that Zeref used most of his power to create END, and hence, END is now stronger and more dangerous than Zeref himself.

According to Marrd Geer, END was created specifically to kill Zeref, because Zeref wants to die; so Zeref making a pact with Igneel (the self proclaimed King of Dragons) to provide someone who can kill him isn't out of the realm of possibility.

As we've seen Lumen Histore is Mavis herself encased in a large crystal. I will posit that the crystal is the Tenrou Jade mentioned in FT Zero. Mavis herself has been shown to be monsterously powerful, so it may be that she sealed herself in it in suspended animation until she could aid Natsu in killing Zeref.

Likely, breaking open the jade will release the insanely powerful Mavis, but most likely she can't be re-sealed. Meaning that she will die after a short battle. This explains why Mavis can interact with the others as a spirit, when no other dead people have been shown to stick around like that.