The character is an ancient, trans-dimensional malevolent entity who preys upon the children (and sometimes adults) of Derry, Maine, roughly every 27 years, using a variety of powers that include the ability of shapeshifting and manipulation of reality.

[1] King thought of a troll like the one in the children's tale "Three Billy Goats Gruff",[2] though he imagined it living in a sewer system rather than under a bridge.

It arrived on Earth during an asteroid impact and made its home under the land upon which Derry would later be constructed, initially preying on North American tribes.

However, It's true appearance is briefly observed by Bill via the Ritual of Chüd as a mass of swirling destructive orange lights known as "deadlights", which inflict insanity or death on any living being that sees them directly.

It's natural enemy is the "Space Turtle" or "Maturin", another ancient dweller of King's "Macroverse" who, eons ago, created the known universe and possibly others by vomiting them out as the result of a stomachache.

[8][9][10] The Atlantic said of the character; "the scariest thing about Pennywise, though, is how he preys on children's deepest fears, manifesting the monsters they're most petrified by (something J. K. Rowling would later emulate with boggarts).

"[11] British scholar Mikita Brottman has also said of the miniseries version of Pennywise; "one of the most frightening of evil clowns to appear on the small screen" and that it "reflects every social and familial horror known to contemporary America".

The character was suggested as a possible inspiration for two incidents of people dressing up as clowns in Northampton, England and Staten Island, New York, US, both during 2014.

[17] Several newspaper articles suggested that the character of Pennywise was an influence, which led to King commenting that people should react less hysterically to the sightings and not take his work seriously.