The circumstances of the Requiem's commission fed a legend almost from the moment Mozart died. In the summer of 1791, while he was finishing The Magic Flute, an anonymous messenger arrived offering a generous fee, half in advance, for a Requiem Mass, on the strict condition that Mozart never learn or seek out who was actually paying for it. The commissioner was in fact Count Franz von Walsegg, an amateur musician with a habit of commissioning works from established composers and then passing them off as his own, who wanted a Mass to commemorate his wife's recent death and likely intended to claim authorship of it himself, as he reportedly did with other commissioned pieces.
Mozart, already unwell and increasingly convinced he was writing his own Requiem in a more literal sense, worked on the score between other commitments through the autumn before his health collapsed entirely in late November. He died on December 5, 1791, with the opening Requiem aeternam complete in full score but the following movements finished only in vocal lines and figured bass, and the Lacrimosa stopping after just eight bars, at what several scholars have noted is a peculiarly apt line of text to have been the last thing he set: "Lacrimosa dies illa," that tearful day.
Constanze Mozart, needing to deliver a complete score to collect the remainder of the commission fee, first approached Joseph Eybler, who completed some orchestration before abandoning the task, then turned to Mozart's student Franz Xaver Sussmayr, who finished the orchestration and composed the remaining movements largely from scratch, producing the score performed under Mozart's name ever since.