1.How did this transformation happen?
- There was literally nothing to trigger it beside the fact that Natsu was getting his butt wooped, he comes out of flames with half of his body covered in scales and he also grew a wing. To me, it didn't seem like there was anything "evil" or "malicious" about END. (Although it was only a partial transformation)
Edit: Actually I lied, there was one feature that was malicious about his transformation, it was his teeth. Looked almost like a shark!
2.Does he use DS Magic or Curses?
- He didn't really say "Fire dragon's ____" and stuff like that, so I assume he was using his regular magic. (Keep in mind that he was perfectly sane, during the time he was half demon-dragonized, he told Animus that his life, body and soul was dedicated to protecting his loved ones)
3.How strong is he?Was he able to defeat a Dragon?
- He wrecked Animus, who was empowered by the Dragon Cry staff in like 2 minutes.
And what happened to Sonya at the end?
- Sonya survives, and she is last seen in the end roll riding a big green bird with a fellow citizen of the Stella Kingdom.