Read the passage.

But there was one thing that was worse. I discovered that the dragon had put a charm on me: No weapon could cut me. I could walk up to the hall whenever I pleased, and they were powerless. My heart became darker because of that. Though I scorned them, sometimes hated them, there had been something between myself and men when we could fight. Now, invulnerable, I was as solitary as one live tree in a vast landscape of coal.

Which common literary theme is best illustrated by this passage?