A long period has passed before Mozart would again turn to the form of piano sonata. In October 14, 1784, a new masterpiece emerges from his hands: Piano Sonata in C minor.
"The instructions for the performance of this Sonata [...] have been lost; they must have constituted one of the most important documents of Mozart's esthetic practice. Did they perhaps contain more personal matters as wel, which had to be hidden from posterity? [...] It is clear that it represents a moment of great agitation, agitation that could no longer be expressed in the fatalistic A minor key of the Paris sonata, but required the pathetic C minor that was to be Beethoven's favorite key for the expression of similar emotions. It has rightly been said that this work contains a 'Beethovenisme d' avant la lettre'. Indeed it must be stated that this very Sonata contributed a great deal towards 'Beethovenism' possible. Contrasting with the concentrated first and last movements, there is a broad concerto-like adagio in the tranquil key of E-flat major, which, in accordance with the true nature of its creator, who could not seek any easy way out, does not lead to a finale in major; on the contrary, the Finale is just as pathetic as the first movement, and even darker."
Here is Sonata in C Minor K. 457, as performed by the great pianist Alfred Brendel.