Idea for the Hellther music: no music. Just some freaky ambiance. I don't know how to explain what I hear in my head, other than like... low "rumbling". The point is, there wouldn't really be any music, but there would be freaky noises. I'm thinking of making the demons all scream when you're in their line of sight.
This is what started playing in my head when I thought about what you might mean by the rumbling...
Not the melody of the music itself; the weird synth sound in the background.