Fairy Tail reveals the 400 Year Plan of the greatest wizard Zeref. The plan was to defeat the strongest of all Acnologia. One day Zeref was collecting herbs and came across the Fire Dragon King Igneel.
Igneel was unlike other dragons who oppose humans. After the discussion, the dragon agreed to teach the dragon slayer magic to Natsu. Five children, including Natsu, were selected by dragons to teach them dragon slayer magic.
Later Zeref met Anna Heartfilia, and they made a plan to send the children 400 years in the future to defeat Acnologia, Humanities greatest threat.
The foster parents (Dragons) of children were sealed inside their bodies. Children were sent to the period which was rich in Ethernano.
Zeref also wanted Natsu (his younger brother) to be strong enough to kill him. Natsu is also the greatest demon from the book of Zeref which is also known as E.N.D (Etherious Natsu Dragneel)