What is the main conflict in It by Stephen King?

It by Stephen King is a novel about friendship and the magic of childhood. The novel centers around seven core characters, friends who bonded as children during a summer of darkness. Twenty-seven years later, they reunite to fulfill a promise made that terrible summer. Evil lives under their hometown and, for some reason they do not understand, these seven have been destined to bring an end to the terrible cycle of death and destruction.