The Demons of Chaos, alternatively translated as Chaos gods or Chaos monsters, are legendary entities in the Monster Girl Encyclopedia world that are sealed away in the deepest part of the sea. They were very briefly mentioned in Monster Girl Encyclopedia I, where their mystery and alleged power nevertheless made them frequent topics of interest. More recently, extra information has been revealed as the profiles for their affiliated monster girls are released.
Wandering Scholar relates the tale that, in ages past, the demons of chaos and Poseidon shared dominion over the sea. But in the age of the former Demon Lord, a war involving the demons led to them being sealed in the abyssal depths. Even today, Poseidon's power and influence doesn't stretch to the deepest parts of the sea. Although the demons of chaos were naturally male and female, they are "kin to monsters", so Wandering Scholar expects that the current Demon Lord's monster-to-monster girls sorcery will have turned them into all-girl nymphomaniac part-succubi too.
This speculation was confirmed in later profiles. The chaos monsters are the masters of the mindflayers and the creators of the shoggoths, although many of the latter fled their service in order to go out into the world and find a husband. In the shoggoth's profile we are also told that their mind-meld style sex where sensations are shared is an "incomplete imitation" of how the demons of chaos mate.
The deep layer is even further towards the bottom of the sea. It's also called the deep sea. It is even further towards the bottom of a giant hole opened in the sea. No light reaches beyond this point. Also, it's a place beyond the extent of Poseidon's rule. Normal monsters and humans never go near it.
According to legend, in the age of the former demon lord, wicked demons of chaos who possessed a power that was even greater than that of the gods existed there at the bottom of the sea. At the time, Poseidon was still one of the divinities ruled by the chief god, and the rule of the sea was split between her and them. It is said that the demons of chaos were sealed in the deep layer after a war, and even now, the demons of chaos are biding their time to make a come back and once again reclaim dominion over the sea.
Those who sunk into the depths of the sea, male and female, are also kin to all monsters, so they've probably assumed the female form, and had their thought patterns changed accordingly, but no one has ever been able to confirm it, and the truth of this is uncertain.
Once a man has experienced this pleasure, he will start to constantly desire their service. The master himself will probably be the one to desire sex, which will involve melding together even more deeply. When having sex with them, it is like one another's genitalia melt together and becomes vague, and so much pleasure is induced that it becomes uncertain which of them is moving or which sensations belong to which person. Through sexual intercourse their master will touch upon their madness, and like them, will desire for the two of them to become one. In this way they will spend their lives having sex by melding together.
It is purported that the ecology of constantly desiring to become one with their man may be an incomplete imitation of the way that their creators, the demons of chaos, have sex with their husbands. Furthermore, it is said that the reason they fled from their mistresses is probably not because they were unsatisfied with their employment, but because they become envious from seeing the demons of chaos become one with their husbands and wished to acquire husbands of their own.