The Great Father is a god worshiped by some members of the Barrows Family, such as Theodore Barrows. Theodore had an abnormal fear of death and worshiped this evil god in order to gain immortality. The Great Father is mentioned in the Clock Tower novel.
The Great Father gave immortality to those who worshiped him and bestowed death on others, but this was an immortality of the spirit, not of the flesh. The idea differed slightly from the endless cycle of rebirth in which the Celts believed in that death of the flesh was necessary to gain this immortality; one gained deathlessness by dying. Through this immortality of the soul, even dead flesh could be animated as if it were once again alive. Terror was what fed this immortality - one escaped death by absorbing the spiritual energy known as terror. The one who bestowed this immortality upon the worshipers was known as a "son of the Great Father" - an entity apparently similar to the angels of Christian monotheism, but unlike angels, a child of the Great Father bestowed only death and terror upon the people.
The dark god demanded sacrifices, and so Theodore abducted children from the surrounding area and, after prolonged torture, butchered them in offering to his god. He soon lost his humanity. Rumors of Theodore's inhuman ways eventually spread among the populace, and, fearing they would reach the king's ears, his relatives poisoned him. It should have all ended then, but the dark god Theodore had summoned continued to haunt the Barrows clan.