Have never read the book, but looked at the summary out of curiosity. It includes the following:

Bill then discovers the "Ritual Of Chüd," an ancient ritual that allows him to enter the "Macroverse" from where It originated. Bill encounters Mataurin or "The Turtle," the creator of the universe and the natural enemy of It, who further explains It's origins.

Bill enters the monster's mind through the Ritual of Chüd and discovers that the true form of It is a mass of destructive orange lights known as the "deadlights," which he repels.

After the battle, the victorious but badly shaken Losers begin to lose cohesion and get lost in the sewers, until Beverly has sex with all the boys to bring unity back to the group.

What

They descend into the sewers, and use their strength as a group to "send energy" to a hospitalized Mike, who fights off a nurse that is under the control of It.

Richie rescues Bill from the deadlights and manages to severely injure It with his Voices.

Bill crushes It's heart between his hands, finally killing It.

https://en.wikipedia.org/wiki/It_(novel)