I'm sure you've heard (of) them already, but if not, check out the Silent Hill soundtracks. Those are truly one-of-a-kind horror soundtracks. Particularly 1-3 if you want some industrial and/or noise madness.
Then there's always
kt2, a kind of obscure (or not?) composer who unfortunately hasn't done much (that I/we know of at least) since 2006 or so. I recommend ABANDONER from him - it's probably his darkest soundtrack. "Nightmare IV" from that soundtrack has a really dark feel to it.
The entirety of the soundtrack "Demento" (aka Haunting Ground) by Seiko Kobuchi is pretty cool I guess. Maybe not the most creative, but if you want crazy, give a listen to
"Something Lacking"...
"Creeping Shadows" from Clock Tower 3 (a "prequel" to Demento) is a favorite of mine in the Clock Tower/Haunting Ground franchise.
Speaking of unique, let's not forget about Drag-on Dragoon. It's more melodic, which may or may not be your cup of tea in terms of horror soundtracks, but I urge you to check out
"Chapter X - On the Ground".
Of course, sometimes you just find dark/unique stuff at random, where you'd least expect it. Actually, something about the Ace Combat 3 soundtrack gives me a Galerians vibe in-between some of the tracks. Like
"Defiled",
"Kernel" and
"Amorphous", just to name a few.. There was something special about the year 1999 for (horror) music I guess (Galerians, Silent Hill 1, and this...)
I don't know, these are all of course just my opinions. It's all I can think of for now. Let me know what you think..