This is far into the war between Alvarez and Fiore. Gray Fullbuster, the ice mage of Fairy Tail has defeated Invel Yura, the Winter General of Spriggan 12. He tells Gray the truth about E.N.D. (Etherious Natsu Dragneel) and how his enemy was right there all along. The reason he hated E.N.D. so much was because he was Zeref Dragneel's creation, an Etherious, and because Gray's father tasked him to kill him as well. However, Invel manipulated Gray overall and told him Juvia died (which wasn't true) so this caused his hatred to grow.

On Natsu's side, however, he was enraged because of a tumor inside him. It started off painful, so it was shrunken down by Brandish. Then she ended up turning the side of the tumor back to normal, therefore having something snap inside of him. His intentions were to end Zeref.

So on the battlefield, they meet up by coincidence. Natsu is completely engulfed in his flames. He even has a dark red demonic arm and claw.

Natsu: "Gray..."

Gray: "Natsu..."

Natsu: "Move."

Gray: "So you were E.N.D. all along?"

Natsu: "Get out of my way!"

Gray: "Answer me, Natsu!"

Natsu: "I'm... going to defeat Zeref... I have to... kill Zeref..."

Gray: "I guess there's no getting through you anymore."

Natsu: "Don't... interfere... I have to kill Zeref! Zeref must die."

Gray then recalls what Invel told him and says

Gray: "I can't believe my mortal enemy was with me all this time..."

Natsu: "Move."

Gray: "My parents... Ur... Everything... You--a demon of the book of Zeref--took it all from me... Even Juvia!" He clenches his fists.

Natsu: "I said move."

Gray: "But don't worry, Natsu. I'm going to stop you now!"

He turns half black, a sign he activated his ice devil slaying magic.

Natsu: "Get out of my way, Graaaaaaay!"

He leaps towards Gray and thus, the fight begins. That's when you, both their closest friend, stumble upon them, surprised that they're actually throwing hands.