Where does this line come from

I Read the first two books and dnf‘d book 3. I don’t think I’ll ever finish it as I know most of what’ll happen because my brother read and told me the story, and the writing honestly just wasn’t for me. Can anyone tell me: what’s with the sentence „Behind them, the heavies rose“? It’s often cited as „the most metal“ or sth of the like, and i get it’s from some battle with theK‘Chain, but why is it so revered?