From the Super Mario Wiki
The Eerie Staircase is a room in the Haunted Towers in the game Luigi's Mansion: Dark Moon. This room is accessed by opening the door on the fifth floor of the Hollow Tree with the Crypt Key. The Eerie Staircase has many dead trees, several spots of foliage, and has twenty flights of stairs with three stairs per row. At the top of these stairs is the Tree House.
During B-Boss: Tree Topping, the Harsh Possessor scares Luigi when he first enters the room, and then immediately flees. Luigi must climb the flights of stairs to the top in order to chase after the Possessor. If Luigi takes a wrong flight, the stairs turn into a slope and Luigi falls all the way to the bottom. Luigi must fight either a Sneaker, a Greenie, or a Slammer if he takes the wrong flight. The blue torch of the correct flight of stairs briefly goes out, or the flames change size, whenever Luigi takes the wrong path, showing him the way to the top. If Luigi takes the correct area, a red torch will light up for future reference. In Hostile Intrusion, the stairs that lead to the top of the Hollow Tree are pulled in, so Luigi cannot come back here.