King Boo revealing the arena.

King Boo's Illusion is an area in Luigi's Mansion 2, and it is the final area in the game, and is the location of the final boss fight with King Boo in his dimension. This area appears to be completely under King Boo's control. It is the smallest area in the entire game, consisting of two "rooms" simply titled "???".

The first room is where the actual fight between King Boo and Luigi takes place. The ground here shifts with Luigi's weight, making objects like the spiked balls that fall from the sky roll after Luigi. King Boo can also make lightning strike the ground, which changes the tiles to have various terrains to hinder Luigi's movement. These can either be ice, sand, or spikes. The area is also bordered by an electrical barrier, which prevents Luigi or the spiked balls from falling off of the platform. This does not harm Luigi in any way. King Boo can also make shock waves in the floor that bounce Luigi away from him.

After Luigi attempts to capture King Boo (to no avail), King Boo will cause Luigi to slide into the second area of King Boo's Illusion. This room is a very long hall where King Boo will chase Luigi down as he tries to dodge obstacles in his way to escape. Luigi will face something from each mansion that he must avoid. This includes: Possessed Armor, lightning that causes dressers to block Luigi's path, moving staircases, spring gears, sand, ice, snow piles, etc. Eventually Luigi will hop on a spring gear that bounces him out of the room, and back to the regular arena.