He is the original Dark Passenger, an ancient dark god who can dwell inside people and influence them to do evil deeds.
Moloch has been in existence since the beginning of time. When humans evolved enough to be aware of his existence, Moloch compelled them to worship him, and to kill each other for his amusement.
At one point, Moloch started to reproduce, but he created too many offspring (passengers), When his children began to rebel against Moloch and be worshiped themselves, Moloch began to kill them off. This resulted in a war between Moloch and his offspring. Moloch was the most powerful and he was victorious. Most of the passengers who would not serve Moloch went into hiding to protect themselves.
In the novel, Moloch's cult members are called Watchers. They sacrifice children inside a bull-headed statue with a fire in its belly where they are burned alive.
Moloch causes Dexter Morgan to hear the music of drums trumpets in order to exert control over him.
Moloch was a Dark Passenger of King Solomon in the Bible.