Palace of Shadow

The Palace of Shadow is the final location Luigi explores in the game. Here, Luigi fights every single ghost he's ever faced so far along with the Shadow Queen and her minions. Luigi needs all seven Star Keys in order to enter the Palace.

Tower of the Star Keys
The Tower of the Star Keys is basically the Altar of the Thousand-Year Door from "Paper Mario: The Thousand-Year Door". Here, Luigi uses the Star Keys to unlock the door that leads to the palace. Luigi needs all seven of them to go any farther. After unlocking the door, Luigi finds himself in the Entrance to the Palace of Shadow.

Entrance to the Palace
The entrance to the palace is where Luigi fights Ultimate Shadow Bouldergeist. After that, he receives the key to the Trick Room.

Ghosts:
Dry Bones

Gold Ghosts

Speedy Spirits

Temper Terrors

Water Elemental Ghosts

Ultimate Shadow Bouldergeist

Trick Room
The trick room is where Luigi has to "follow the flames" to get to the next room where he must fight Ultimate Bogmire. After beating Ultimate Bogmire, he receives the key to the Bullet Bill Room.

Ghosts:
Dry Bones

Blue Twirlers

Speedy Spirits

Blue Blazes

Ice Elemental Ghosts

Ultimate Bogmire

Bullet Bill Room
The Bullet Bill room is a dark room that goes down where Luigi will find a door that leads to his fight with Ultimate Shadow Boolossus. After beating him, Luigi receives the key to the Balcony.

Ghosts:
Dry Bones

Purple Punchers

Speedy Spirits

Flashes

Fire Elemental Ghosts

Ultimate Shadow Boolossus

Balcony
The balcony is where Luigi fights Ultimate Shadow Bowser, Ultimate Bonetail, Hooktail, and Gloomtail and receives the key to the Entrance to the Tower of Riddles.

Ghosts:
Dry Bones

Bowling Ghosts

Ceiling Surprises

Purple Bombers

Garbage Can Ghosts

Speedy Spirits

Ultimate Shadow Bowser

Ultimate Bonetial

Hooktail

Gloomtail

Entrance to the Tower of Riddles
Here, Luigi fights Ultimate Bonechill, Ultimate Dry Bowser, and Ultimate Shadow King Boo and receives the key to the Tower of Riddles.

Ghosts:
Ultimate Bonechill

Ultimate Dry Bowser

Ultimate Shadow King Boo

Tower of Riddles
The Tower of Riddles holds no bosses, but a few enemies. Luigi must solve all eight riddles to go any farther. However, in order to activate the mechanism at the top of the tower, Luigi must go into the room at the far right of the entrance to the tower and beat Ultimate Shadow Luigi and Ultimate Shadow Mario to receive a star-shaped key that allows Luigi to fit the stone keys into eight pedestals to unlock the stairs that leads Luigi towards the final fight.

Ghosts:
Dry Bones

Dull Bones

Dark Bones

Red Bones

Ultimate Shadow Luigi

Ultimate Shadow Mario

Dark Hallways
These hallways are the last area Luigi ventures through before confronting the dread Shadow Queen herself. Here, Luigi must defeat, Ultimate Shadow Kingfin, Ultimate Shadow Doopliss, Ultimate Shadow Koopalings, and Perfect Dimentio.

Ghosts:
Gold Ghosts

Purple Punchers

Blue Twirlers

Temper Terrors

Flashes

Blue Blazes

Garbage Can Ghosts

Ceiling Surprises

Purple Bombers

Bowling Ghosts

Speedy Spirits

Dry Bones

Dull Bones

Dark Bones

Red Bones

Ultimate Shadow Kingfin

Ultimate Shadow Doopliss

Ultimate Shadow Koopalings

The Shadow Queen's Throne/Shadow Queen's Chamber.
The Shadow Queen's Throne is the final area Luigi ventures in the Palace of Shadow and is the sight of the first part of the final battle. after Luigi beats the Shadow Queen the first time, he must follow her down a long stair case and into the Shadow Queen's Chamber where the second part of the final battle takes place. Here, Luigi only fights the Shadow Queen.