Although wrote Mozart the great "Fantasie" in which he achieves a unique blending of funeral in C Minor with demonic chromaticism – six months after the Sonata, he endorsed the intrinsic connection between the two works by publishing them together. They appeared under the title "Fantaisie et Sonata pour le Forte-Piano" in December 1785 with a dedication to Madame Therese von Trattner. Madame von Trattner (1758-1796) was a pupil of Mozart and the wife of Mozart's wealthy and influential landlord. It has been suggested that an unhappy love affair with her might have provoked the serious expression of both works. However, no proof for this assumption exists.
This Fantasy might well be ranked as Mozart's most significant single piano composition. Like the Sonata in C Minor, it opens in unison, like the Concerto in C Minor, it is chromatic. The chromatic steps around the fifth (F-sharp, G, A-flat) convey a sense of impeding doom, as they do in the main subject of the Concerto's first movement. The Fantasy seems freely constructed, but is in fact most tightly knot. The overall shape is a succession of slow, fast and again slow sections, with a return of the first theme towards the end. The chromatically descending bass in measures 10 16 supports bold harmonies, with an almost Schubertian modulation from C Minor to B Minor. The two slow lyrical sections are in D and in B-flat Major, i.e. the supertonic of C and its mirror. It is worth noting that due to the intense modulations, this Fantasy has no key designation, but is seemingly written in C Major.