This nocturne displays a solemn, tragic character that contrasts sharply with the gentler works in the cycle. It is distinguished by its heavy, somber left-hand chords that support a deeply expressive, weeping melody.
The biggest challenge is controlling the density of the sound to avoid muddiness in the complex harmonic textures.
Comparable difficulty and character — good candidates for what to learn next.
Nocturne in C minor, No. 1 is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 8.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Nocturne in C minor, No. 1 lasts about 3.5 minutes.
Nocturne in C minor, No. 1 is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Nocturne in B-flat minor, No. 1 by Frédéric Chopin, Nocturne in C-sharp minor by Frédéric Chopin, Prelude in D-flat major, No. 15 by Frédéric Chopin — see the full similar-pieces list on this page.