Mozart's Adagio in F minor (K. 411) is likely a movement or standalone piece, often described as deeply expressive. Its distinctiveness lies in its rich harmonic language and emotional depth.
Requires careful control of voicing to bring out the inner voices and harmonic tension.
Adagio (K.411) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
Adagio (K.411) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.