I decided to write another prelude/fugue combination, so here's the prelude. It's a bit melodically complex for a prelude, so maybe it's a bit more of a nocturn-ish thing in itself, but it goes nicely with the fugue, and it does kind of explore the key signature.
Check out the fugue that goes with it here:
https://flat.io/score/63019cc8598c66001221ad3d-fugue-in-d-minor