This is (most likely) the last piece Bach ever wrote (or dictated from his deathbed, according to his son, C. P. E. Bach): a chorale prelude based on the chorale melody Wenn Wir in höchsten Nöthen sein The chorale melody has four phrases, which you'll be able to recognise in the organ prelude. Each phrase of the melody is preceded by a new fugal exposition (with the second subject entry in inversion, which is why it might not sound like a regular fugue), accompanied by an ensuing fugal development and then rounded off with a cadence/short coda. You'll hear this happen four times, once for each phrase. The chorale melody stands out from the texture, but if you listen to it played on the organ, the melody is played on the other keyboard so it stands out even more, but I particularly like the brass version. I think it has a nice timbre (quite suitably sombre).