This occurs during your second Dark Prince transformation (this is in the large courtyard right before the first chariot sequence). In the corner of the courtyard, there will be a hole in the ceiling that respawns enemies until you leave the area. Near that is a panel you need to stab and hang from to open the exit. Climb the closest pole to the hole in the ceiling and a female voice will start announcing sound credits for the game.

During gameplay, pause and input RIGHT, LEFT, RIGHT, LEFT, DOWN, DOWN, UP, UP, Y, X, Y, Y, X, X.

During gameplay, pause and input UP, DOWN, UP, DOWN, LEFT, RIGHT, LEFT, RIGHT, YELLOW, BLUE, YELLOW, BLUE.

During gameplay, pause and input LEFT, LEFT, RIGHT, RIGHT, Y, X, X, Y, UP, DOWN.

During gameplay, pause and input UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT, Y, X, Y, X.

Finish the game.