For 10 points, name this work, left incomplete at its composer's death, a mournful mass written by the composer of Eine Kleine Nachtmusik.
A corner to the penultimate page of this piece's score was stolen at the 1958 Brussels World's Fair. The second violin plays quarter notes alternating with the first violin's eighth notes in the first two 12/8 bars before the vocalists enter in another section of this work. Count von Walsegg commissioned this piece and likely intended to pass it off as his own. Its "Sanctus," "Agnus Dei," and all but about nine bars of its "Lacrimosa" were the product of Franz Sussmayr, who completed this piece at the behest of its composer's wife Contanze. For 10 points, name this work, left incomplete at its composer's death, a mournful mass written by the composer of Eine Kleine Nachtmusik.
A corner to the penultimate page of this piece's score was stolen at the 1958 Brussels World's Fair.
Wolfgang Amadeus {Mozart’s Requiem} Mass in D minor [prompt on partial answer]