The Ritual of Chüd was a battle of wills and was the only way to defeat It.
Novel
Bill Denbrough first found the information about the Ritual when he found Night's Truth in Derry Public Library, where he also found It was a Glamour known to many cultures under many different names. The Ritual itself is from Himalayan belief, who recognized It as a sort of taelus. In the Himilayan tradition, a holyman and the taelus overlapped tongues, bit in to each other, and told riddles until one laughed despite the pain. If the taelus laughs first, it gets sent away for a hundred years, while if the man laughs first the taelus gets to eat the mans soul.[1]
As children, Bill is the only one to engage with It, being thrust toward the Macroverse, heading to the Deadlights, but his physical body remains put. He speeds by the Turtle, who only offers advice that "–you must help yourself, son," and "you've got to thrust your fists against the posts and still insist you see the ghosts[...] once you get into cosmological shit like this, you got to throw away the instruction manual."[2] Bill engages with it telepathically, biting his teeth into Its' tongue, saying "He Thrusts his fists against the posts and still insists he see the ghosts" in his fathers voice repeatedly. Overall, the fight is one of the Losers optimism, imagination, unity, and belief over Its malice and anger. The Losers come out victorious, but ignore the Turtle's advice to make sure they finish the deal, and It escapes, which the Losers suspect but are not sure of.
As adults, Bill is the first to engage It again. However, without his childlike imagination, he is weaker in the battle. It taunts him, saying that the Turtle died some time ago. Bill 'misses' Its tongue, and Beverly calls out that something is wrong, It is laughing. Richie quickly realizes something is wrong and screams out in his Irish cop voice, catching Its tongue and being thrown into the universal sprawl with Bill. He saves Bill from the Deadlights, threatens It with his Voices, but they still struggle against it. As before, their bodies remain still in the real world, but Eddie hears Richie calling for help, and rather than enter with them, he uses his aspirator as before to seriously hurt It in the physical world, losing his arm in the process and dying of blood loss. In this time It is able to escape further into her lair, dropping eggs along the way that Ben stays to crush, as Beverly remains with Eddie's body. Reluctantly Richie leaves Eddie and Bill leaves Audra to go further after It needing to ensure It dies this time. Finding It, they hit It with their collective belief and love and childhood nostalgia along with the power of the Other[3]. Richie is knocked out, Bill crushes Its heart between his hands, and carries Richie, who he believes may be dead, back to the other Losers.
It (1990)
Bill Denbrough first battled It with the Ritual of Chüd with advice that was given to him by Maturin. The ritual was a psychic battle in which the two forces dueled with their wits. The children believed that the metal silver had supernatural abilities, as seen in numerous monster movies. Because the children believed it, the silver became real and was a chief weapon that was used in the ritual as children. Because Beverly was good with a slingshot, the Losers' Club injured It the first time when Beverly shot a chunk of silver into Its skull. The Losers thought that they killed It, but weren't sure, so they made a pact to return to Derry should It ever return. It was finally destroyed in the second Ritual of Chüd by the adult Bill, Richie Tozier, Beverly Marsh, Eddie Kaspbrak (he was killed by It) and Ben Hanscom.
It Chapter 2 (2019)
Here Mike Hanlon discovers the Ritual of Chüd himself by visiting with local Native American tribes as an adult. The Ritual involves burning tokens special to all those in the Ritual to expose Its true form as the Deadlights and trapping them in a vessel Mike stole from the tribe. Unlike the novel, the ritual is unsuccessful, as the Native Americans that attempted to use it to trap It failed and were brutally slaughtered. Mike withholds this truth from his friends, believing that the tribe was unsuccessful because they had not truly faced their fears and felt that his own group stood a far better chance at survival and victory. The Losers' performing of the Ritual exposes the Deadlights, but fails to actually contain It and are forced to back off. Taking the form of a half-spider/half-Pennywise hybrid, It separates the Losers to go through personal trials, overcoming each together giving them strength. However, this exposes Richie to the Deadlights causing Eddie to attack It with a spear, dealing a serious blow. It retaliates, killing him over Richie, but not before Eddie is able to tell the Losers they have to make It small to kill it. The Losers succeed in doing this, convincing each other that Pennywise is small by insulting It, until all the remaining Losers crush Its heart together, finally killing the ancient evil.