I was playing a mod for Oblivion called Dungeons of Ivellon and let me tell you, it's one of the scariest things to play. I'm not a guy who gets scared easily, I can play Slenderman just fine and Amneisa as well as others, but this...
Despite being alot shorter than Amneisa (it's only a mod after all) I think that it's scarier simply because the history behind Ivellon (The name of the buried castle) is much more sinister and the enemies are much more diverse as well as being more grotesque.
Oh and it's pitch black. Those torches you see lit are ones I lit when I came to them.
If you own Oblivion for PC, then I recommend this mod. Have fun sh*tting your pants!
And the enemies are not pre-set. They spawn in when you reach certain points. That means there is absolutely no way to detect them ahead of time.