Confront the Source
From the Super Mario Wiki, the Mario encyclopedia
Unlike all other missions, Luigi starts in the Lobby instead of the Entrance. In this room, he should investigate the elevator with the key, which makes him fall into the Cellar. If Luigi tries to escape the Lobby, E. Gadd will contact him, demanding that he has to get to the Cellar. When Luigi arrives in the Cellar, he should go down the stairs, which suddenly retract into the floor and cause him to slide to the bottom. Luigi should then attempt to vacuum the web in front of him, disturbing the large spider on it and starting the boss fight.
Luigi has to make use of nearby fires, attacking suits of armor, and webs to reveal the Grouchy Possessor; once the Grouchy Possessor has been shocked three times, it is defeated and the mission is cleared.
This paranormal reading is off the charts! I don't know what's down in that Cellar, but it's not happy. Why don't you...uh...investigate?
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