Day of wrath! Day on which
The world will disintegrate
into glowing ashes
As foretold by David and the Sibyl!
How great the quaking will be
When the judge comes
And strictly examines everyone!
Dies iræ! dies illa
Solvet sæclum in favilla
Teste David cum Sibylla!
Quantus tremor est futurus,
quando judex est venturus,
cuncta stricte discussurus!
Full lyrics (though musically, only verses 1-2 are known as the Dies irae; the other verses of the original poem are broken down into separate pieces, such as Rex tremendæ, Confutatis, and Lacrimosa.)
Answer: Dies Irae, from the Requiem Mass. (Lyrics probably by Thomas of Celano, 13th century) Many composers have written music for the Requiem Mass, but the two most widely known instances of the Dies irae the version by Giuseppe Verdi and the version by Wolfgang Amadeus Mozart & Franz Xaver Süssmayr. (Guessed by dolique and sigma7)
Where you've probably heard it: Verdi's version: It's frequently used in trailers for movies with a lot of explosions; HBO used to used it (and may still) to advertise their summer action movies. Mozart's version: in the movie Amadeus. Both--but especially Verdi's--are commonly used wherever someone wants to evoke a sense of imminent (or already upon us) apocalypse.
Verdi's version, version 1 (this video cuts off about 20 seconds early, though)
Verdi's version, version 2 (this video isn't quite as good, but includes the whole piece and the very beginning of the next, as the songs in Verdi's Reqiuem blend from one to the next)
Feudalism: Serf & Turf