I'll bet you thought it was Requiem for a Dream, didn't you? Nope. It's Lux Aeterna. A piece originally by Mozart, Lux Aeterna was used as the central theme in the film Requiem for a Dream, and has been used so often since then there isn't a person alive who has had any exposure to modern entertainment who hasn't heard this song. However, people always call it Requiem for a Dream after the film, but they're wrong, although this particular version is actually a remix called Requiem for a Tower, created for the second Lord of the Rings film. Anyway, Lux Aeterna. Brilliant piece