Requiem in D minor, K.626

Mozart had been commissioned to compose a requiem by Count Franz von Walsegg to be performed on the anniversary of his wife Anna's death on February 14 of 1791. Left incomplete upon Mozart's untimely death on December 5th the same year, Mozart's widow Constanze (anxious to receive the last half of the payment for the commission) entrusted the completion first to Joseph von Eybler - who returned the manuscript in frustration after working on the Dies irae through the Lacrimosa. Constanze then assigned the task to Franz Xaver Sussmayr who finished the task so the score was dispatched to the count in the Spring of 1792 complete with his forged signature of the composer dated the same year.
Walsegg had a reputation for passing off the works of other composers as his own and was no doubt planning to do likewise with Mozart's work. He was not the only person to receive a copy of Sussmayr's completion, for Constanze sold one to the King of Prussia in March of 1792. Despite the stipulation in the commission that the work not be published, Constanze ultimately sold copies of the work to Breitkopf & Harel and Johann Andre. Walsegg was additionally upstaged for the premiere by a benefit performance given on January 2, 1793 in Vienna for Constanze.
The generous-format study score offered here is a newly engraved and thoroughly researched edition by Richard W. Sargeant, Jr. based upon the Mozart's holgraph fragment, Sussmayr's completion and other primary sources.

IMSLP page
Wikipedia article