Here's a recording of an incredibly complex Ricercar (a sort of Fugue, see https://en.wikipedia.org/wiki/Ricercar
) by the one and only J.S. Bach.
It is the highpoint of his Musical Offering (BWV 1079) and the theme was given to the composer by Frederick the Great (back then king of Prussia).
Some say that Frederick wasn't too fond about the old fashioned Baroque music, so there is a possibility Bach wrote this hardcore Baroque composition as a more or less cynical response to the king's musical preferences...
The transcription for organ has been made by German organist Herbert Collum (1914-1982) and in the attachment you'll find the score. There are some mistakes in it, so be aware.