King Boo's Illusion

King Boo's Illusion is the final area in Luigi's Mansion: Dark Moon. It is where Luigi battles King Boo. Upon defeating the Tough Possessor from the Paranormal Dimension for the first time, Professor E. Gadd tries to pixelate Luigi out, to meet him in the Bunker. However, the transmission is intercepted by King Boo, and Luigi is placed back into the illusion world, automatically starting the next mission, A Nightmare to Remember. After completing the game's story mode once, King Boo's Illusion will be available on the mansion select screen.

In-Game Description
''King Boo has created the ultimate illusion, and he's waiting for you to show yourself. Are you brave enough to face him in his own territory once again?''

Trivia

 * During the intro cutscene to King Boo's Illusion, Luigi is confronted by the portrait of Mario while accompanied by a remix of the Super Mario Bros. theme.