Gardner, John. Grendel.

But there was one thing worse.  I discovered that the dragon had put a charm on me: no weapon could cut me.  I could walk up to the meadhall 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 solitary as one live tree in a vast landscape of coal.