It was a piece that was comissioned. It was also not completed by Mozart before he died. From: http://en.wikipedia.org/wiki/Count_Walsegg-Stuppach Mozart had been commissioned anonymously to write the Requiem (by intermediaries acting for the eccentric Count Walsegg von Stuppach). Count Walsegg-Stuppach was the aristocrat who, in 1791, sent a messenger to Wolfgang Amadeus Mozart to commission a requiem mass. The count, apparently an amateur musician, had a penchant for commissioning works from composers of the day and then passing those works off as his own in private performances. Walsegg-Stuppach intended to have the requiem performed (as his own composition) in memory of his recently deceased wife. Mozart died, however, before completing the requiem

The work was commissioned by one Count Franz Walsegg-Stuppach, via the infamous "grey messenger." Walsegg-Stuppach was himself an amateur composer (being a composer was seen as a middle-class occupation, so a nobleman would never have done more than dabble in composition as a hobby), but one who had a penchant for commissioning works from the great composers of his day (as well as having had the money to do so), with the intent of putting his own name on them and having them performed as his own works. When Mozart died before completing the work, his widow Constanze was forced for financial reasons to have the work completed. Not only would this insure complete payment of the commission, but it would avoid repayment of the advance that had already been paid. After unsuccessful attempts at completion by Franz Eybler, she authorized Mozart’s student Franz Xaver Süßmayr to complete the work. Although other completions have been written, it is Süßmayr's which is most often performed. One other note on the inspiration for the Requiem: there is a myth popularized by Peter Shaffer's play "Amadeus," and the subsequent movie, that Mozart saw the "grey messenger" as a harbinger of his own death, and thought of the Requiem as his own funeral mass. In truth, symptoms of Mozart's final illness had not yet begun to present when the commission was made. in other words, Mozart did not yet know he was sick, let alone dying. It is more likely that, in his own mind, he actually thought of the work as a Requiem for his father, composer Leopold Mozart, who had died only three years earlier.