There's an untitled song at the end of the Danzig's fourth album that sounds like a Satanic chant. There's a bunch of blank tracks before that one, which is the 66th track.
I recall Marilyn Manson had a hidden track at the end of a couple of their CDs, one of them was a horrifying bit where some distorted hideous voice is saying "Come on, play with me, step to me, what game do you want to play? Do you want to play poop game? Do you want to play broken glass against the head game?" That one is hilariously brutal and perverted!