Fišer's eight Piano sonatas have a special place in his oeuvre. Fišer subsequently eliminated his second sonata (1956) from his compositional repertoire. From the third sonata onwards (1960), subtitled Fantasia, the composer wrote a two-movement composition, in which he continued to incorporate as his fundamental musical device the confrontation of sharp contrasts in tempo and mood. Beginning with his fourth sonata (1962–1964), Fišer created a single-movement work in an expressive, formally focused composition which betrays a progression towards greater compactness of musical shape in a concise yet effective musical testimony. The fifth sonata was written in 1974, the sixth sonata in 1978. The seventh sonata from 1985 was dedicated to František Maxián, the eighth sonata was written in 1995.
Of a total of eight Piano sonatas in Fiser's oeuvre, the sonata with its subtitle Fras (Old Slavonic word for devil ) is his sixth; it was written in 1978, a period when the composer, after his creative exploits of the 1960s, returned to a more traditional manner of musical expression and notation. Beginning with the fourth sonata (1962-64), Fiser created a one-movement piece which clearly shows a development towards greater density of musical form and a concise, eloquent testimony. Piano Sonata No. 6 is also conceived in one movement, constructed on the principle of contrasting sections and gradation in order to achieve a fitting dramatic effect.