The Fallen God is a deity of the Monster Girl Encyclopedia world who has been corrupted by demonic energy, transforming into a monster. Her subordinate lesser divinities have become dark angels and dark valkyries, while her gospel is promulgated to mortals primarily by dark priests. If it were possible, the Fallen God's religion manages to be even more depraved than monster girls usually manage; "They want to thoroughly corrupt men so that they'll come at them and ravish them with endless desire" . Although the Fallen God is allied with the Demon Lord, their agendas do not precisely align; like Zipangu, her monsters do not contribute to the Demon Army. Instead, they preach a doxa of complete, unrestrained depravity, and delight especially in corrupting prudish or restrained targets.
Monsters who follow the Fallen God consider semen a sacred liquid, and love having it sprayed all over them. This means they tend to wear (if anything) black robes, so that semen shows up better. It also gives them a preference for the Corruption Fruit, improving their ability to absorb spirit energy through the skin. Ironically, it's the disloyal followers that get into the Fallen God's heaven: when they find a man to be their husband, they desert their missionary duties and (somehow) enter the spirit realm of Pandemonium, to fuck forever in a world without time.
The Fallen God can perform a special kind of monsterization by usurping the Chief God's connection with devoted priestesses. This happens in Fallen Maidens to Sasha Folmoon, where she is spoken to by the Fallen God in a religious dream. Although the Fallen God commands the loyalty of the dark valkyries, they "have a business-like attitude when dealing with her" , as they consider their husbands to be their true masters.
A group of monsters ruled by the corrupted divinity that is called “the fallen god” who have Pandemonium as their citadel. At present, only the “dark angels” are in the encyclopedia. 
They were originally divinities, but they were changed into monsters after being attacked by monsters. It's an organization of those who have fallen and drown in pleasure. Although they are divinities who were changed into monsters, they're a force that's neither grouped with the “divinities” nor grouped with the “monsters.” In the same way as with the other gods, it is comprised of a greater divinity “the fallen god” at the top, and her subordinates, the lesser divinities “dark angels," dark valkyries" etc. and the "dark priests" who are human women converted into monsters by them.
Now let's get back on course after that crazy story from before.
There are many other beings called gods besides just the chief god.
The chief god is trying to fix the system that the demon lord threw out of whack, but it seems not all of the gods think that way.
“The Fallen God” was consumed by devilish pleasure, corrupted, and changed into a monster while still possessing the power of a god. For the sake of the denizens of the sea whom she loved, “Poseidon, god of the sea (p. 128)” joined them and went on the demon lord's side. The alliances of the gods are never fixed and unchanging. Presently, the demon lord has several divinities as allies. The chief god is in a position where it's getting harder and harder to strike at the demon lord.
A faith that is governed by the “Fallen God”, a high rank god who became completely depraved and received demonic energy in her body, that believes the true form of living things is when they're drowning in pleasure and falling. The devotees of the Fallen God are humans and monsters that fell through pleasure and chose the path of falling even further. It's regarded as virtuous to expose all of ones desires or catch all of a partner's desires with ones body. Conversely, concealing desire and enduring through abstinence is regarded as taboo. For that reason, as for the subordinates of the Fallen God, “dark angels”, and her worshipers “dark priests”, when they spot puritanical humans, they thoroughly pleasure them and make them fall. If it's a human woman, they'll change her into a “dark priest”.
The female devotees’ desire is to aim their lewd desires at men, and have men aim their lewd desires at them. As far as they're concerned, the thick milky white stuff released by men is desire itself, an extremely sacred liquid. As for their black robes, they wear them because they like having their entire bodies showered with semen, and the white stands out most against the black, so they can strongly feel themselves being sullied, painted, and covered in thick milky white desire. As for those who awaken to the teachings of the Fallen God, many of them awaken to the faith when they are changed into monsters by having sex with her devotees and the doctrine is instilled in their bodies and hearts, but when adherents of the Chief God are changed into “succubi”, it is said that sometimes they're able to hear the voice of the Fallen God directly in their hearts, and they awaken to the faith of the Fallen God after being guided by that voice.
Eventually the monsters devotees of the Fallen God who obtain a human man head for their utopia, the Fallen God's holy land and castle, “Pandemonium”. Due to the power of the Fallen God, time is distorted in that land. It is said to be a space where the same moment continues eternally and there's no aging or growing hungry. There they are promised days of having sex with men forever consumed by unfathomably deep desire, and a bottomless fall with continuous indulgence in pleasure.