I really wanted to do this..... Like, I SUPPORT THE DRAGNEEL BROTHERS!!! (Cuz I'm one of them... :P) Onto the summary!
During a seemly regular night, a hurricane struck. As Natsu was running home in the forest without Happy, since said Exceed stayed with Wendy, he found someone he had not expected hiding under a tree. Zeref. Of course Natsu didn't trust him, but he can't stand seeing someone-even if it was an enemy-get drowned by the rain. So, Natsu took him in. After a few days of nursing the Black Wizard to health from the fever he had caught in the rain, seeing that Zeref didn't have anywhere to go, Natsu let him stay. Fairy Tail seems to accept this until much later, but when they did... Acnologia came into the fray.
Dragneel Brothers Story