This is kind of an unusual piece for Bach. He wrote dozens of fugues in different styles and keys, but the passacaglia is unique. The passacaglia is a musical form from Spain and it's usually of a serious character and is often based on a bass-ostinato and written in triple metre. In Bach's piece the passacaglia is introduced as an ostinato of eight bars played by the pedal that is repeated during the first section, and it continues with increasingly complex variations on the theme.
The passacaglia is followed, without break, by a double fugue. The first half of the passacaglia ostinato is used as the first subject; a transformed version of the second half is used as the second subject.[14] Both are heard simultaneously in the beginning of the fugue. A countersubject enters immediately afterwards and is then used throughout the piece. When the three subjects appear simultaneously, they never do so in the same combination of voices twice.
The fugue form as expressed by Bach is the highest form of baroque music, if not all music: complex, beautiful, transcendent, and never again to be seen since the baroque era of music.
There doesn't seem to be anything here yet