Dance with the Devil (found unreleased Prince song; 1989)
Originally created for the famous 1989 Batman film, this song was based around the Joker's famous catchphrase/question "Have you ever danced with the devil in the pale moonlight?". It was chosen to be included the the Batman album, after Batdance, but was ultimately removed because Prince felt it would have been to dark of a follow-up song.
It was allegedly never "officially" released, but it was used as promotional material before the movie was released.
It was also originally posted to YouTube and music sharing sites, but all traces of it were deleted with cease and desists from Warner.
EDIT: Here is the song in question, found from the 2011 Prince bootleg collection, Work It!
The quality is not the best, being in MP3 and sounding like it was recorded externally, but it is still very listenable though, way more than other bootlegs.