The Requiem in D Minor is perhaps one of Mozart's most powerful and poignant pieces still standing today. The requiem, intended for the memorial mass of the wife of Count Franz Walsegg-Stupach, was written while Mozart himself was dying. It's been debated over the years, decades, probably even centuries, how Mozart's death came to pass; some say it was murder by poison, various other sources cite pneumonia, and yet other sources say that he died of a venerial disease. The official belief is that he died of the miliary fever which he was already afflicted with when he began to compose the Requiem. Whatever the case may be, the last Requiem written before his death stands by itself as an emotionally powerful piece.
It has been questioned as to exactly how much of the Lacrimosa Mozart managed to write himself before his death; some say more than half was written by Mozart, other sources point to a belief that says he wrote the first eight bars of the piece before his death, afterwards stating that it was likely that one of his students, Franz Sussmayr finished the rest of the Lacrimosa, and most likely the rest of the requiem. This is most usually made as an assumption, though, and not stated as actual fact. It has, however, been proven that up until the day of his death, Mozart was able to sing the lyrics of the Lacrimosa to the students that he was still instructing at the time, usually joining with several friends or students to sing the not yet finished Requiem.
The Lacrimosa itself was said to have brought Mozart to tears upon having heard it sung by the small choir he was working with. The fact that the choir was singing the Lacrimosa, most likely in full, suggests that perhaps Mozart had finished much more of Sequenz IV than most people give him credit for.