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The Tree House is a room inside of the Haunted Towers in Luigi's Mansion: Dark Moon. This room is accessed by completing the Eerie Staircase. In the Tree House, there is a large arena, a staircase, and a house, although this house is never entered.
During B-Boss: Tree Topping, the battle with the Harsh Possessor occurs in this area. After defeating the Possessor, Luigi readjusts the stairs and walks in front of the house where the Dark Moon piece landed.