It is the title character in Stephen King's 1986 horror novel. The character is an ancient cosmic evil which preys upon the children of Derry, Maine roughly every 27 years, using a variety of powers that include the ability to shapeshift, manipulate reality and go unnoticed by adults. During the course of the story, It primarily appears in the form of Pennywise the Dancing Clown.
Pennywise has a preference for children since their fears are easier to interpret and adults are harder to scare. It can manipulate people with weaker wills, making them indifferent to the horrific events that unfold or even serve as accomplices.