A deeply expressive nocturne composed for the composer's sister, later published posthumously. It is characterized by its soulful cantabile melody and demanding, improvisatory-style passagework.
Achieving seamless voicing between the melodic line and the delicate, shifting accompanying figures requires advanced control.
Comparable difficulty and character — good candidates for what to learn next.
Nocturne in C-sharp minor (Op. posth.) is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 9.
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-sharp minor (Op. posth.) lasts about 4 minutes.
Nocturne in C-sharp minor (Op. posth.) 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 Waltz in C-sharp minor, No. 2 by Frédéric Chopin, Étude in E major, No. 3 by Frédéric Chopin, Nocturne in B-flat minor, No. 1 by Frédéric Chopin — see the full similar-pieces list on this page.