"Requiem" was the last composition of Mozart

Explanation

Requiem in D minor, K. 626, was his final composition left unfinished at his death on December 5, 1791. It was commissioned by the count Franz von Walsegg to memorialize the passing of his wife and was finalized by Franz Xaver Süssmayr, and potentially other composers, in 1792. It became famous because the commission was anonymous (Walsegg wanted to pass it off as his own work) and because Mozart thought he was composing his own Requiem due to being very ill.