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His Requiem, composed in 1837 is a radical setting of the Catholic Mass for the Dead. It is an enormous work, requiring a huge choir, large orchestra and four specially placed ensembles of brass instruments, used to stunning effect. The work is not performed often, but it influenced opera composers Richard Wagner and Arrigo Boito when it came time to announce the presence of divine influence in their operas.
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Ideas from the Requiem also show up in Mefistofele (1868) the only completed opera by Italian composer and critic Arrigo Boito. Mefistofele is a setting of Goethe's Faust (which Berlioz set in 1846 as La damnation de Faust.) Boito and Berlioz both made their livings as music critics, and each composer wrote difficult works that met with hostility at first, but were eventually accepted into the repertory.
Mefistofele opens with a scene set in the heavens, depicting the Devil's famous wager with God. The stakes: Faust's soul. God is offstage, represented by singing choirs of angels (also offstage) echoed by huge slablike chords in the brass. This idea of trumpet choirs, echoing back and forth with the main orchestra, shows up in the most famous moment in the Requiem. In the Dies Irae section of the mass, (which depicts God's wrath and the Last Judgement) the brass choirs announce the Tuba Mirum--the blowing of the last trumpet. In both works, the effect is unforgettable.
Watch the Prelude to Parsifal here.
Watch the opening scene of Mefistofele here.
Top: Hector Berlioz. Middle: Richard Wagner. Bottom: Arrigo Boito. All photos from Wikipedia.