The Fourth Doctor Regenerates

Having sustained a fatal fall from the Pharos Project radio telescope, the Fourth Doctor assures his companions that "it's the end, but the moment has been prepared for." Upon saying this, the Watcher, a manifestation of the Doctor's future who'd been following the TARDIS team around throughout the serial, appears and merges with the Doctor, allowing him to regenerate. As this happens, the scene is underscored by a medley of the Watcher's leitmotif and the show's title theme, representing the "death" of the Fourth Doctor and the "birth" of the Fifth. The end of Four's era (and the end of Season 18) is further hammered in by applying an electronic blur to Tom Baker's face in the closing credits.

Example of:
Leitmotif upon Death