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It (also commonly known as Pennywise) is an ancient alien/eldritch monster and the title character and main antagonist of Stephen King's best selling and award winning 1986 novel. 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.