It (Stephen King)

Pennywise is the name of the evil demonic clown used as the main avatar of the mysterious creature known as "It" in the film and book of the same name.

He is an infamous child-killing monster that targetted children for a long time until finally being defeated by a gang of kids - however Pennywise swore revenge on them for his defeat and returned to kill them off as adults.

In the climax of the film Pennywise is killed again - however in the final-battle he takes the form of a spider-like monster rather than the clown disguise which he used for most of the film.