I don't know about "frightening," but the anime(s) that had a decent impact on me were as follows...

Ninja Scroll Resurrection
This is among the craziest things you will watch animated. There is no justice... no good guys win. Just messed up... plain and simple.
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Wicked City
Another delightfully twisted series involving demons and sexual themes.
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Apocalypse Zero
Another messes up series.
Pretty immature... but will leave an unpleasant memory I'm sure.
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There are many more (the names escape me atm) but most of the real twisted stuff you'll find in the 90's era. Believe me when I say today's stuff pales in comparison.

The plural is "anime" btw.

Oh yeah... and "Nightmare" from Robot Carnival.
This kind of bothered me as a kid. I was able to find it on youtube... which is kind of awesome.

http://www.youtube.com/watch?v=7oGxbCc0Xq8

Also, does anyone remember that "Alien" wannabee series? The one where that girl's cat gets mutilated? That one was pretty messed up too. I still haven't forgotten it to this day.

Kazuo Umezu's a guy whose works are a great blend of weird/creepy/gory/hilarious, not to mention an eccentric personality to match it. Whether you've heard of him or not, I'm sure you've come across at least one "Umezu-Face" (pic related) if you've seen enough anime.

Sadly, there's only one animated and english-subtitled work of his and that's The Curse of Kazuo Umezu (http://www.bakabt.com/details.php?id=144283&page=0#comments). It's decent enough (I suppose I would've been scared by it if I were younger) but really, it's a much better use of your time if you read one of his manga instead (Left Hand of God Right Hand of Devil for something short, Drifting Classroom for something longer).

Not anime, but anything by Junji Ito.

…OP here, just a suggestion: don't waste your time watching Kurozuka.

>>6158
I know how you feel. I watched Shiki believing that I would get a good horror anime, but got homolust, plotholes, and bad forced drama.