Pennywise The Dancing Clown
"It" apparently originated in a void containing and surrounding the universe. Its real name (if, indeed, It has one) is unknown — although at several points "It" claims its true name to be Robert Gray — and is christened "It" by the group of children who later confront it. Likewise, Its true form is never truly comprehended. Its favorite form is a clown (with fanged teeth and large clawed hands when it stalks a child) called Pennywise, and Its final form in the physical realm is that of an enormous spider, but even this is only the closest the human mind can get to approximating Its actual physical form. Its natural form exists in a realm beyond the physical, which It calls the "deadlights". As such, the deadlights are never seen, and Its true form outside the physical realm is never revealed, only described as writhing, destructive orange lights. Coming face to face with the deadlights drives any living being instantly insane. The only one person is known to have escaped the "deadlights".

"It" believes he is the "eater of world and of children." For millions of years It dwelt under Derry, awaiting the arrival of humans, which It somehow knew would occur. Once people settled over Its resting place, It adopted a cycle of hibernating for long periods and waking approximately every 27 years. Its waking spells are marked by extraordinary violence, which is inexplicably overlooked or outright forgotten by those who witness it. Both its awakening and return to hibernation mark the greatest instances of violence during its time awake.

1715 – 1716: It awoke.
1740 – 1743: It awoke and started a three-year reign of terror that culminated with the disappearance of over 300 settlers from Derry Township, much like the Roanoke Island mystery.
1769 – 1770: It awoke.
1851: It awoke when a man named John Markson poisoned his family, then committed suicide by eating a white-nightshade mushroom, causing an excruciating death.
1876 – 1879: It awoke, then went back into hibernation after a group of lumberjacks were found murdered near the Kenduskeag.
1904 – 1906: It awoke when a lumberjack named Claude Heroux murdered a number of men in a bar with an axe. Heroux was promptly pursued by a mob of townsfolk and hanged. It returned to hibernation when the Kitchener Ironworks exploded, killing 108 people, 88 of them children engaged in an Easter egg hunt.
1929 – 1930: It awoke when a group of Derry citizens gunned down a group of gangsters known as the Bradley Gang. It returned to hibernation when the Maine Legion of White Decency, a Northern counterpart to the Ku Klux Klan, burned down an African-American army nightclub which was called "The Black Spot." One of the survivors, Dick Halloran, appeared in King's earlier novel, The Shining.
1957 – 1958: It awoke during a great storm which flooded part of the city, and murdered George Denbrough. It then met its match when the Losers forced It to return to an early hibernation when wounded by the young Bill Denbrough in the first Ritual of Chüd.
1984 – 1985: It awoke when three young homophobic bullies beat up a young gay couple, Adrian Mellon and Don Hagarty, throwing Mellon off a bridge (echoing real life events in Maine). It was finally destroyed in the second Ritual of Chüd by the adult Bill Denbrough, Richie Tozier, Beverly Marsh, Eddie Kaspbrak and Ben Hanscom.

In the intervening periods between each pair of events, a series of child murders occur, which are never solved. The book's surface explanation as to why these murders are never reported on the national news is that location matters to a news story—a series of murders, no matter how gruesome, don't get reported if they happen in a small town. However, the book's implied reason for why the atrocities go unnoticed is far more sinister: It won't allow them to be.