What was Mozart’s last, incomplete work?

Mozart’s “Requiem” in D Minor, K 626, is a requiem mass or mass for the dead, that was left incomplete at his death on December 5, 1791. It was commissioned by Count Franz von Walsegg-Stuppach, as a tribute to his wife’s death.

At the time, Mozart was deeply engaged with the writing of two operas: “The Magic Flute” and “La Clemenza di Tito” (“The Clemency of Titus”). Together the three assignments were too much for a man suffering from a succession of debilitating fevers. Most of his failing strength went into the operas, both of which were completed and staged.

As for the Requiem, he worked on it when strength permitted. Yet his condition worsened, and, by the time of Mozart’s death early on December 4, 1791, he had finished only the “Introit.” The “Kyrie,” “Sequence,” and “Offertorium” were sketched out. The last three movements—“Benedictus,” “Agnus Dei,” and “Communio”—remained unwritten, and nearly all the orchestration was incomplete.

Mozart died of “severe miliary fever” or “rheumatic inflammatory fever". Other diagnoses, considering his history, included Schönlein–Henoch syndrome.

Mozart’s wife, Constanza, aware that there was a fee due at its completion, handed the manuscript to Joseph Eybler, who supplied some orchestration but was reluctant to do more. Then to Mozart's student, Franz Xaver Süssmayr, who produced a complete version, writing several movements himself though possibly basing them on Mozart’s sketches or instructions.