#1
I'm looking for ways to create claustrophobic and suffocating music for a project I may be concieving in the near future (possibly as part of my solo project or a full band, that's undecided).
So I need to know how to create music that's crushing and suffocating (metaphorically of course :p along the lines of Portal, Ruins of Beverast and such.

Also, influence won't only be bands, films, books and such will also come into play.
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#2
well let me take out my theory textbook and look up the chapter on claustrophobic music.....

seriously? I can't tell you that, its not a black and white thing--the reaction to music is dependant on the person. I love free jazz, but to many people it sounds like it doesnt make sense and can be painful to the ears, likewise music like Miley Cyrus drives me up a wall.
Think about what you find suffocating or clatrophobic. Maybe its dissonance, lots of syncopation, total atonality and tons of voices (which is what I'd try). Experiment and experiment until you compose something that gets the mood you want across, but you can't be taught to compose something to match your own perceptions, thats something you learn on your own (you can however, obviously be taught to compose music that goes along with set classical and popular music standards). If I was writing something like this, I'd investigate the music of schoenberg (sp?) and other twenty first century composers, the music of don ellis and possibly some debussy (the later more dissonant stuff, but even that is extremely beutiful), thelonious monk even a band like Dillinger Escape Plan.
#3
Study those types and use your ear - if you are looking for theory in this, then you need to have basic harmony and diatonic understanding solid, so that where it deviates, you can follow along with it. For example, if I start talking about b2 and you know tiritones, b5s and such, you would need to have a basis for understanding it. So in your case, if you don't have that, and Im assuming by the question that you do not, study the bands you like, emulate them, develop an ear and then, build from it. That's all you have available, if you cannot analyse and study from a theory standpoint itself. Anything that departs from Diatonic Harmony, is more advanced than Diatonic itself.
#4
listen to a bit of Lifelover; polish avant-garde, depressive black metal music.
and watch the Merhige film "Begotten".

from what you have said, i am relating both of these to the emotions and themes you have described.
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#5
Wall of sound production, low frequencies, loud dynamics, compression, tight bass. Not a lot of ambience/reverb/echo. Make it punchy. I dunno what else to say.
#6
Using odd-time signatures helps. Especially when the rhythms do not shift smoothly or naturally, providing a very awkward and uncomfortable aura.

Comfortable music provides a lot of well-controlled tension and release that isn't too extreme. If you were to ignore one of those aspects completely and amplify the other to an extreme amount - the music would be unbearable to most (a suffocating aura). For example you have a song that has a constant tension in building up to a grand point, but a release never occurs. It just stops. That can be pretty unnerving. Or you have a song like this that is just a constant extreme release with no tension or build-ups.

A song gets hairy after a while when there is no tension without release, or release without any preceding tension.
#7
TC you should check out Gorguts - Obscura. Its some canadian death metal, its got some very uncomfortable textures and is pretty much all atonal stuff. Alot of subtle free jazz and 20th century classical influences. The main songwriter also wrote some Schoenberg style classical.