Wikipedia:
The Imperial March or "Darth Vader's Theme" (Episodes I, II, III, V and VI). The theme that represents the totalitarian Galactic Empire as a whole, and Darth Vader specifically. More than other Star Wars themes, the March has attained an iconic status in the Western consciousness as a general "evil theme", and as such is used to portray power at public events, sometimes seriously, sometimes with tongue in cheek (as in sporting events). It has been used on multiple occasions to introduce a scene featuring the "evil" Montgomery Burns on the animated comedy The Simpsons. Musical features include relentless martial rhythm and dark, non-diatonic harmonic support. In the Original Trilogy, The Imperial March also represents all that is the Empire; therefore, it is nearly equivalent to a galactic anthem. Williams retrograded the theme for the prequel trilogy, subtly embedding it in Anakin's innocent theme and the evolution of the Republic (represented by the clone troopers) into the Empire. It is heard with progressive prominence through Episodes II and III, signaling critical points in Anakin's downward spiral to the Dark side. In the March's final rendition, accompanying Vader's death in Episode VI, Williams reverses the effect of the theme, by means of reduced orchestration and volume. It ends with a cadence of solos (strings, flute, clarinet, horn and, ultimately, harp) as Vader expires.