Fantasia In D Minor, K. 397 - Adagio
Fantasia K. 397 is a work for solo piano composed in 1782, though at the time of Mozart's death, the piece remained incomplete.  The original manuscript was never found, and its unfinished bars are believed to be completed by August Eberhard Müller. The Fantasia is one of Mozart's most unusual works, hence its name.  It's full of unusual rhythms, seven changes of tempo, and almost entirely loses musical form (something beyond the realm of normality during the classical period).