From the Super Mario Wiki
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 enters the portal to find the last Dark Moon piece, Mario and King Boo. The portal leads to the Paranormal Dimension, where Luigi battles the Tough Possessor, as well as King Boo. After Luigi has defeated King Boo, and returns to his own dimension, the paranormal portal disappears.
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