This fugue from Book I of the Well-Tempered Clavier is known for its intense chromaticism and complex subject. It is highly regarded for its dense, emotionally profound development of the fugal subject.
At Henle 6 this is an advanced piece that assumes fluent technique and rewards mature interpretation. The main demands: independent voices that must each sing on their own line; fine finger independence within the hand; and precise articulation — the difference between crisp and blurred is the piece.
Beyond that, expect chord voicing — bringing out one note inside a handful of others.
Maintain complete clarity of the four distinct voices while managing the dense, chromatic interplay.
Comparable difficulty and character — good candidates for what to learn next.
A level or two down with the same spirit — ideal if this piece is still out of reach.
Where this piece's skills lead: slightly harder works making similar technical demands.
Lesser-known works by the same hand, at a comparable level.
Das Wohl Temperierte Klavier, vol I, Fugue 18 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 Das Wohl Temperierte Klavier, vol I, Fugue 18 lasts about 4 minutes.
Das Wohl Temperierte Klavier, vol I, Fugue 18 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 Partita No. 2 in C minor by Johann Sebastian Bach, Concerto and Fugue by Johann Sebastian Bach, Concerto in G minor by Johann Sebastian Bach — see the full similar-pieces list on this page.