From the Super Mario Wiki, the Mario encyclopedia
The paranormal portal is a purple portal that appears on the Terrace on top of an altar, in the Treacherous Mansion in the missions Paranormal Chaos and Stop the Knightmare. When created by King Boo, this portal releases a large hoard of Ghosts into the mansion. Luigi must capture all of these ghosts as quickly as possible to lower the paranormal readings and prevent his dimension from collapsing upon itself. Afterwards, Luigi is sucked into the portal, which leads to the Paranormal Dimension, where Luigi finds the last Dark Moon piece and Mario, and battles the Tough Possessor and King Boo. After Luigi has defeated King Boo, and returns to his own dimension, the paranormal portal disappears.
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