My first thought was Ride of the Valkeries by Richard Wagner. But I guess it's so overdone now, and used for so many cartoons and satires, that it doesn't have the dramatic appeal that it once had. Sigh.
But I still like it a lot!
I also think that Bach's good ol' Tocatta and Fugue in D minor is very "apocalyptic-sounding!"
Frankly, I think a lot of the music from the Nutcracker Ballet could be used. I'm not talking about the 8 pieces in the Suite, but more of the ballet score. There is a lot of dark music in Nutcracker.
And of course, there's the wonderful Night On Bald Mountain by Mussorgky! Yes, again, it's overdone, but there's a reason why music is overdone--because it's good music and people like hearing it! Boy, that piece of music sure sounds like the Book of Revelation!