Didn't see this posted, by far the darkest song I know off the top of my head
When I saw Rob Zombie in concert and he played this song, I felt like going to church the next morning.
And one of the heaviest songs I've heard not written by Slayer, A7X's 'God Hates Us All.'
Seems like the heaviest songs are the ones that start off soft, and then BAM! EXTREME HEAVINESS!