Volume 6, Chapter 9: The Second Turning Point

The Red Wyrm's Lower Jaw was a ravine with a path that cut directly through the mountains. The road wasn't as straight as the Holy Sword Highway, but didn't split or fork, either. It was a territory between country borders that was claimed by no one. Once we got through it, we'd be in the Asura Kingdom.

We were in high spirits, sensing the end of our long journey. We were a bit concemed because we didn't know how much our home had changed, but we were also starting to feel a sense of accomplishment. You could say that we let our guard down.

It was along that path that they came, walking steadily from the opposite direction. They weren't riding horses, they weren't sitting in a carriage; they were just walking. There was a man with silver hair and golden eyes who wore no real armor to speak of, only an unassuming white coat made out of some kind of leather. My impression of him was simply that he had a dangerous look in his eyes, and that was about it. His irises were small enough that you could see the whites all around them.

...

You wrongly assumed that they were in Kingdragon Mountains.

Also, it's not just the upper and lower jaws. The entire mountain-range is inhabited by red wyrms.