Nightfall

Hiccup had a friend once, when he was 10. They were never separate from each other. But one day, that friend started going into the woods, every afternoon. And one day, never came back. Stoick sent out a search party, but there was no trace of the boy. Nothing.

5 years later someone arrives on Berk, they're 15. They top everyone in Training. But they're hiding something.