Read the Haunted Staircase clues
Use the special key to enter the Hollow Tree. Each staircase junction has three choices, and the route changes between attempts. Look at the blue flames: the correct stairs have an extinguished or smaller flame. Follow that visual cue instead of memorizing a left-right sequence. Wrong choices send Luigi down and summon a ghost; previously solved choices become marked with yellow flames. At the top, pull the hanging cord three times to break through, then use the next cord to lower the ladder to the Tree House.
Defeat the Harsh Possessor
The apparent staircase at the Tree House is the Possessor’s host. Dodge its burning jumps and slams until it lands and pauses with the fire gone. Move behind it and pull the red tail to overturn it. Stun the exposed Harsh Possessor and vacuum it, filling the Power Surge to break its protection. It returns to the staircase after the first two captures. Stay clear of the fire, wait for the next exhausted landing and repeat the tail pull. The third exposure lets you capture it and recover the Dark Moon piece for the Old Clockworks.