The Great Father is believed to be a double-strength flowfiend from the Outer Planes. He is capable of creating new flowfiends from evil or chaotic neutral humanoids that his minions capture and bring to a remote "island" in the phlogiston (made entirely from the calcified remains of beings that have fallen into the flow). There is an altar on the island, and flowfiends can summon the Great Father by placing a victim on the altar and whistling for 1d10 hours. If the victim survives a system shock roll 1d4 turns later, he or she is transformed into a flowfiend, becoming chaotic evil in alignment and utterly loyal to the Great Father.
The location of the Great Father's lair is unknown, as are his exact reasons for creating the flowfiends.