It crashlands on Earth in a cataclysmic event similar to an asteroid impact, landing in Northern America, on the spot that would eventually become Derry, Maine. Every 27-30 years, It would awaken from hibernation to kill and eat, before going back to sleep to repeat the cycle. He eventually starts taking on the form of Pennywise the Dancing Clown.
1740-1743: It awakens to start a three-year reign of terror, preying upon over three hundred settlers who had founded the Derry Township shortly before. In the end, all three hundred settlers are simply gone, not even their bodies are left.
1876-1879: It awakens, then returns to sleep after murdering a group of lumberjacks.
1904-1906: It awakens, when a lumberjack named Claude Heroux massacres several men in the Sleepy Silver Dollar bar, after they murdered his friends and fellow lumberjack unionists. Shortly after, Heroux was lynched by an angry mob. It's rampage ended with the Kitchener Ironworks explosion, where 108 people, 88 being children, died during an easter egg hunt.
1929-1930: It awakens, when Derry's residents and police ambush the Bradley Gang and gun them down Bonnie and Clyde style. Pennywise actively participates in the slaughter. The town pretend it never happened, and the gang were killed by the FBI. In 1930, the Maine Legion of White Decency, a northern equivalent to the Ku Klux Klan, burn down the Black Spot, an African-American nightclub. Mike Hanlon's father, and a young Dick Holloran, are amongst the survivors. It appears as a giant bird trailing balloons. It returns to hibernation.
1957: It awakens, when Dorsey Corcoran, brother to Eddie Corcoran, is beaten to death by his stepfather Richard Macklin. It kills numerous children, starting with Georgie Denbrough. Seven children - Bill Denbrough, Ben Hanscom, Beverly Marsh, Eddie Kaspbrak, Richie Tozier, Mike Hanlon, and Stan Uris - form the Losers Club, and decide to kill It to stop the slaughter. It manipulates bully Henry Bowers to try to stop them, but fails. The Losers Club wound It using the Ritual of Chud, but it escapes to return to hibernate. The seven children make a promise to return to Derry should It return in twenty-seven years time. Bowers is institutionalised, blamed for the child murders of 1957.
1984-1985: It awakens, when three bullies beat up gay couple Adrian Mellon and Don Hagarty. Mellon is thrown off a bridge, and Pennywise kills him. It begins killing people. By the time It has murdered nine children, Mike Hanlon, now a librarian and privately investigating Derry's connection to It, decides to summon his friends to fulfil their pact. All but Stan return, who commits suicide at home. It releases Henry Bowers to murder the Losers Club, but Bowers is accidentally stabbed by Eddie. Bill, Ben, Beverly, Eddie, and Richie confront It in Derry's sewers, discovering Bill's wife Audra has been taken prisoner. It appears as a gigantic, nightmarish spider, killing Eddie. The others attack It and successfully destroy him using the Ritual of Chud.