Book One, Fairy Tail - Outcome

My name is Zeref Dragneel. I'm your brother.
Natsu. My beloved brother, has finally understood. He will finally kill me.
Thank you, Layla, Anna, Mavis...Natsu.
Zeref's words. If you kill me, you will die yourself.
...
It's been a year since The Final Battle between Natsu and Zeref. It has been one year since both of their deaths.

The anniversary of Natsu's passing was coming up...it seemed to be a dire day before a firey explosion. And from the dirt of the ground he was buried, rises Natsu Dragneel, holding his opponent Zeref.

Why did they come back?

( no parings, unless interpreted)