I am about to link to a video that should be the stuff of nightmares. Perhaps it isn't, but it should be.
But honestly, to be truly horrifying, you need to do some things to prepare first:
1. Arrange to listen to some scary music or background noise, something other than the soundtrack to the video itself (because the video's music is just strange). Try to pick something that evokes a horrible death, an inevitable doom that you cannot escape from, or an evil that is almost beyond comprehension. Perhaps something alien.
2. The first view of the video is looking down on a bunch of round objects. When you see them, do your best to convince yourself that you're looking at the heads of your adventurers, protruding from the ground, as if they are buried up to their ears. When the video shifts to the side-view, again, convince yourself that you are looking at a human, dwarven, or elven head, possibly with lifeless (or perhaps drugged) eyes staring blankly at you.
When viewed when following the above pieces of advice, I think you will find that the video is pretty terrific, in the most horrific ways imaginable.
Here is a link to the video. Watch it.
I personally found the first two minutes or so to be the best, but you might feel otherwise.
I found the video in this article, but I highly recommend watching the video before bothering with the article.
Finally, if anyone can point James Raggi in the direction of this post, I'd appreciate it. I think that he would really enjoy it. (I used to be listed on his 'Other Stuffies to Read' but he probably removed me after months of my not posting.)