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It (also commonly known as Pennywise) is an ancient alien/eldritch monster and the titular main antagonist of Stephen King's best selling and award winning 1986 novel, the 1990 miniseries of the same name, and the film series of the same name. It appears in SMG4 as a form of Gooigi.
He kidnaps Luigi, so Mario, SMG4, Toad, and Fishy Boopkins goes on an adventure to save him. After they went inside a well, he says that he is Luigi. He then transforms into Gooigi and proceeds to attack the gang and then SMG4 sucks It with Poltergust 3000.