I'm looking for music about the horrors of war, be it metal, classical, punk, whatever. The only prerequisite is that the music be terrifying sounding. Something that sounds like a caravan of blind mustard gas victims moaning in pain. I don't really know how to describe it any better way.

I can recommend two bands that have a lot of parallels with each other:

Zyklon-B - Blood Must Be Shed
Uranium 235 - Total Extermination

Both of these bands were short lived and released very little material, but they do seem to fit your request quite well. Their sounds can be described as ripping primitive black metal about war and genocide and all that wonderful stuff. Also, I'd recommend you check out Benjamin Labarrère's (Uranium 235 dude) other project Godkiller. Doesn't really fit this request so much, but the material he made in that project is amazing and some of my favorite metal.

God Dethroned's Passiondale and Under the Sign of the Iron Cross are exactly what you're looking for. They're concept albums about WWI, and they perfectly depict the terrible consequences of the war through a relentless blackened death metal assault, which is also permeated with occasional bouts of melancholic melody and an epic scope. Out of the two, I'd say that Under the Sign is the superior one (and it also has what might be the best GD song ever), but both are worth your time.

Dammit! I was going to rec these guys, although I believe Passiondale is the superior album. I dunno, Under the Sign sounded a bit redundant and recycled to my ears in comparison albeit still a very good album.

Anyway, OP, I rec you KYPCK. Very dreadful, sombre doom from Finland with a few former Sentenced members. Very crushing stuff, pretty well all of their lyrics revolve around Soviet history and WWI.

http://www.youtube.com/watch?v=7QP765PRJKE
http://www.youtube.com/watch?v=pbzRG48FCa4

Obvious reccomendation is Discharge! Every song is usually about the terrors of war and nuclear apocalpyse. Other bands like Discharge talk about this kind of thing especially the Japanese band Disclose. Also check out No Fucker as well.

For classical: http://www.youtube.com/watch?v=Dp3BlFZWJNA

While Sabaton mostly sing about battles on a positive achivement sense, they do have quite a few songs talking about the horrors of war too. They may be do your liking.

If Sabaton is 'terrifying' enough, then the obvious Sodom must be recommended too.

I'm going to recommend Hail of Bullets - the caustic, rough and crushing tone of the guitars, and the intensity of the music in general really gives a sense of flames, carnage, twisted metal and the pounding of artillery. The more melodic sections can really convey the grief, loss and tragedy of war, too.

Five Finger Death Punch are a great band for this subject as well. Allot of the album "War is the Answer" deals with allot of post war horrors and what soliders go through.

Sodom - M-16

This is a very good rec, and I endorse it.

I'd also like to mention a band that was recently brought up in conversation in the heavy/power/speed thread; Armor Column. They're absurdly vicious power/thrash with insane vocals. Seriously, think a mix of Artillery, Overkill, and Jag Panzer with what sounds like a hybrid of Bobby Ellsworth and Harry Conklin behind the mic, and you got it down almost to a T. They're very technical too, and their debut (and only album to date), Maximum Collateral Damage, is centered exclusively around the devastation created by war. In short; they sound like a fucking blitzkrieg made music.

Im a big fanboy for this band but their latest album is among the best concerning war, death, and madness.

http://www.metal-archives.com/albums/Ca ... ver/336090

Ancient Rites' song "Ypres" might be sth you looking for. More eerie than terrifying but you could give it a chance:
http://www.youtube.com/watch?v=0weAHh-4oVg

Furthermore you could try the Martial Industrial project Un Defi D'Honneur. Both of their albums have WW1 as a theme.
http://www.youtube.com/watch?v=aJ4i6wU3NDU
http://www.youtube.com/watch?v=iKunQZrMvRw