Mozart's Requiem, perhaps more than any other of his works, is shrouded in myth and legend. The subject various adaptations that vary from the speculative to the outrageous, its real story is an interesting enough one in its own right.
In the late 1780s and early 1790s, Mozart was working in Vienna and was in financial difficulties. In what were to be the final weeks of his life, he was commissioned to write a Mass for the Dead. The commissioner wished (for reasons that will become clear) to remain anonymous, and therefore sent an envoy to communicate with the composer. The unannounced visits of this shadowy individual seem to have reminded Mozart of a messenger from the next world, and he became obsessed with the idea that he was writing his own funeral music. He was seized by a feverish need to complete the work before his own death.
The commissioner of the Requiem was in fact the Count Franz Walsegg-Stuppach, a keen but dishonest amateur musician who had the habit of passing other composers' works off as his own. It is known that he went so far as to copy out the title pages of music he had bought, replacing the composer's name with his own! The Count's wife had died in 1791, and he intended to commemorate her with a musical work by Mozart for which he would once again claim the artistic credit. Sure enough, after the Requiem's publication he concocted an implausible story in which he, supposedly a pupil of Mozart, sent his Requiem to the composer for approval, only to have Mozart hold onto the sketches until after his death, which - so the Count wanted people to believe - led to the legend growing that Mozart had composed it!
Aside from this attempted hijacking, the other curious quality of the Requiem is its incomplete nature. Mozart died before finishing all the movements, and various other composers (and, later, Mozart scholars) sought to reconstruct the movements to fit what they believed Mozart would have written. The first, and most influential of these, was Sussmayer's completion. Again, the story has been somewhat elaborated to make it more dramatic, with a myth arising that the rival composer Salieri, a mediocre composer consumed by jealousy of the younger man's genius, actually poisoned him. (Mozart is now believed to have died of a disease that can cause a metallic taste in the mouth, which would explain his own stated fears of foul play.) This story has been the subject of a play by Pushkin, an opera by Rimsky-Korsakov, and more recently the film "Amadeus".
It is difficult to be sure how much of the Requiem is Mozart's own work. Sussmayer made several boastful claims when he sent the completed version of the work to the publishers, in which he portrays himself as having a close working relationship with the composer and therefore unrivalled insights into his thinking as regarded the Requiem. In a few instances, however, he has ignored or even overruled the sketches left by Mozart, and the role of other "assistants" seems to have been played down in order to create the impression that Sussmayer was the only person responsible for taking on the great man's legacy.
More recent attempts to strip off all additions and leave just the music that is known to be by Mozart have been made; there are now numerous alternative completions, but Sussmayer's is so common as to be effectively canonical.
There is a further interesting tale connected with the Requiem which, for once, does appear to be backed up by documented facts to an extent. At the 1958 World's Fair in Brussels, the autograph of the score was on display, evidently not under very secure conditions. At some point during the fair, someone was able to gain access to the manuscript, tearing off the bottom right-hand corner of the second to last page, containing the words "Quam olim d: C:" (an instruction that the "Quam olim" fugue of the Domine Jesu was to be repeated "da capo", at the end of the Hostias). As of 2011 the perpetrator has not been identified and the fragment has not been recovered. Current Mozart scholarship suggests that this instruction may have been the last words Mozart wrote before he died; the thief, at any rate, must have believed so.