"Light from the Netherworld," the background music to the opening movie. Incredibly eerie and chill-inducing. Even if you've never even seen the opening movie, you can tell just by listening to it that something inexplicable and unearthly is going on.
And now the Orchestral version is out, edited into the Opening FMV.
Id's theme, "The One Who Is Torn Apart." It can only be described as haunting.