Be aware there are spoilers for IT Chapter Two (2019), IT (2017), the 1990 miniseries, and Stephen King's novel.
We all know Pennywise is terrifying. Whether you grew up on Stephen King's novel, Tim Curry's iconic performance in the 1990 miniseries, or just met the dancing clown via Bill Skarsgard's unique but equally terrifying performance in Andy Muschietti's two-part film adaptation, the consistent through-line is that the creature known as It is an ungodly manifestation of our most primal fears. And one creepy clown. But what exactly is It? Well, that's not super easy to answer, but let's give it a try.
In both the book and the films, It is an ancient alien creature, older than civilization, and in King's novel, older than our universe. It feasts on the flesh of humans simply because our fears are easy to manifest and they make us taste better. According to It, when humans got scared, "all the chemicals of fear flooded the body and salted the meat". This is why he prefers to feast on children -- their fears are simple, pure, and powerful compared to the complex, pathological fears of adults. Basically, children are delicious. That hunger for tasty, tasty, beautiful fear is pretty much the sole reason It returns to Derry, Maine every 27 years to torment and feed on the townsfolk before retreating into a new cycle of slumber.
Its physical form lives in our world as a shape-shifting manifestation of your worst nightmares, but in King's novel things get much, much weirder, Its true form is a Lovecraftian, ambiguous demigod that lives in the so-called Macroverse -- an unnameable and unknowable malevolent force that considers itself Eternal. It's mortal enemy is a great Turtle, a fellow resident of the Macroverse who, according to It, accidentally belched up our universe in a fit of indigestion. It considers the Turtle inferior, but the only other creature near It's status. The Turtle is believed to be Maturin, the same benevolent god-creature that factors heavily into King's 'The Dark Tower' series. After the Losers Club bands together and manages to hurt It for the first time in Its existence, throwing off the predictable cycle of safety It has always known, It begins to fear for the first time there may be an even greater Other.
Beyond Its physical form lies what It calls the deadlights, a sea of destructive orange lights that drive most humans insane upon sight. In the books, Bill nearly glimpses the deadlights and survives, but the only person to fully see the deadlights and recover is his wife Audra. In the film, it's Beverly who glimpses the deadlights when Pennywise unhinges his jaw and reveals a peek at his true form. We also see subtle hints of the deadlights when Pennywise's eye glow orange throughout the film, first when he terrorizes Mike and again when he retreats to the Well House after the Niebolt street battle.
When Beverly is shown the deadlights, she instantly goes catatonic and dead-eyed, floating in Pennywise's cistern until Ben resurrects her with a classic true love's kiss, but once she does, she recovers quickly. In the films, glimpsing the deadlights don't seem to have the same devastating effect that it does in the novel, but there were some interesting side effects. In IT Chapter Two, we learn that Beverly was changed by that encounter, even when she couldn't remember it. She already knew how Stan died, finishing his wife's sentence on the phone, and as she explains later, she has seen them all die in her dreams every night since leaving Derry. Those turn out to be visions of the fates that await them if they fail to fight against It.
Richie also gets a full blast of the Deadlights in Chapter Two, instantly stopped dead in his tracks, he starts to float slack-jawed in the air. Once the connection is broken, however, Richie also recovers quickly, unlike Audra's in the novel, whose catatonic state lasts long after she and Bill return home after the battle with It. For Richie, it's unlikely that there would be lingering fallout from seeing the deadlights like there was for Beverly since its influence seems to dissipate at the end of Chapter Two, allowing the Losers to remember each other this time.
As you probably would expect from a celestial creature, It's powers are not restricted to embodying your worst nightmare driving people insane with his space lights, but It is best known for Its shape-shifting abilities. Pennywise is only one of Its forms. In the film, we also see It become a mummy, Beverly's dad, Mike's burning parents, the creepy painting lady, a decapitated boy, a leper, and Georgie, and in the book, he takes many more forms, most famously, the classic Universal monsters.
But It can also appear as much grander and stranger spectacles. In the books, he attacks Mike as a giant bird, swooping through the remains of the Ironworks. Other forms include a swarm of piranhas, winged leeches, and of course, a giant spider laying her eggs in the Derry sewers. Muschiettie embraced that weirder side of Its manifestation in Chapter Two, where we saw the living 30-foot Paul Bunyan statue, the nightmarish creatures that crawl out of the fortune cookies in Jade of the Orient, and the monstrous forms teased during the hallucinatory origin sequence.
It can also manipulate people into violent action, or sometimes, inaction that allows violence to continue. We see this in the film when Henry Bowers murders his father and sets out to kill the Losers, when the car drives by and leaves Ben helpless during Henry's torment, and in the way It has spread through the town's history like a cancerous corruption. In the book, Pennywise's evil deeds are writ large in histories, flashbacks, and knowledge passed down to Mike from his grandfather.
In the first film, we get a glimpse of It's far-reaching influence through Ben, who takes over Mike's role as the resident history nerd. Ben explains Derry's dark history, telling us that people die or disappear at six times the national average in their town... and that's just the adults the kids are worse. "Way, way worse." First, we learn about the Ironworks which inexplicably exploded in 1908 killing 102 people, including 88 children who were participating in an Easter egg hunt. We also hear about the Black Spot, a night club created by and for local black soldiers that was burned to the ground by a hate group in 1962 (1930 in the book).
Ben also passes on the tale of the charter for Derry township, which started as a beaver trapping camp in the 18th century (per the book). It crashed to the ground there thousands of years prior, but It only began to feed when the Derry settlers arrived. All 91 settlers vanished, a la Roanoke, with no explanation. There were stories of a battle with Native Americans, but no signs of an attack. All that was left was a bloody trail leading to the Well House. As we later learn, the Well House is located in 29 Niebolt Street, where It lives.
There's also a quick reference to the Bradley Gang in 1935, and a bit more details can be seen on the wall mural outside the butcher shop (where the Losers tend to Ben's wounds). The Bradley Gang were an infamous group of bandits who robbed and killed multiple Derry shop owners before the townspeople rallied against them and shot them dead in the streets. In the novel, that story is passed on through creepy pharmacist #1, Mr. Keene and the book is filled with many, many more instances of how It has corrupted the history and town of Derry, from eating children to inciting violent chaos, Its fingerprints are all over the dark history of the town.
In Chapter Two, Mike is the man with the intel, the only Loser who stayed in Derry and kept his memory, and who spent the last 27 years digging into Its mythology. Mike's research leads him to a Native American tribe that live outside the Derry town limits -- outside of Its reach -- and there, he has a vision of It arriving on earth, falling from the sky and crashing into the Earth's ground in the land that would become Derry. In that vision, we also glimpse the Ritual of Chüd, which is said to defeat the creature. In the book, it's a truly insane process that involves metaphorically biting down one each other's tongue and telling jokes until the loser laughs, and it also sends Bill and Richie into the Macroverse.
In Chapter Two, the ritual involves the collecting of totems from their forgotten childhood memories, which are then destroyed and It is meant to be collected in a basket. But the ritual doesn't really work, and as Pennywise reveals, it never worked for the tribe either -- he slaughtered them all. Ultimately, It is defeated by the unity of belief and display of power from the Losers. Chapter Two continually reinforces the theme that "all things must abide by the rules of the shape they inhabit" (also true for the mythology of the book, where it's werewolf form was vulnerable to silver, etc.) That's why Eddie can choke It and hurt It while It's in the leper's form.
With that in mind, the Losers try to force SpiderCrabPennywse into a smaller physical form, but when that plan fails, they realize they can make It feel small, draining It of Its power by taunting It with Its weaknesses. When Pennywise shrings down to the roughly the size of a giant baby, Mike reaches into Its chest and pulls out Its heart, which the Losers crush in their hands.
