Ruddigore or "The Witches Curse" was a major come-down for Gilbert and Sullivan when it opened in 1887. Unfortunately it followed The Mikado, a hard act to follow by anyone's standards.
In Ruddigore, all of the bad characters become good and the good ones become evil - a parody of more traditionally black and white themes where the heroes are all truly heroic and the villains complete dastards.
Of course the music and lyrics are what make any G&S operetta what it is, one patter trio from the opera is My eyes are fully open which is kind of cute because it is also self-referential, and kind of unusual because it is the only G&S patter song which is a trio.