At the end of Star Wars Episode III: Revenge of the Sith, while Anakin/Darth Vader is getting his new "suit", what song is playing? It is like a slowed down version of the Imperial March.
Here's a video clip of the scene:
It's called "The Birth of the Twins and Padme's Destiny". Here's the track as it was released in the film's official soundtrack:
John Williams did many slowed down, subtle versions of Vader's theme in the prequel trilogy so as to hint at and build up to Darth Vader's rise to power.
The official soundtrack differs slightly from the film version of the music (for whatever reason), here is the film-version of the music which includes the low strings playing Vader's theme slowly just after Vader dons his mask: