It was formed after the birth of the Three Supreme Gods. After the residual souls became Rathowm, he escaped from the Gods and entered a space unknown to them, one in which he could hide inside and conserve his power without any divine intervention from the Creator.
The only God that knows the location of Rathowm's Devil Realm is Supreme God Harmonitt, but he keeps that information concealed because he finds the situation both entertaining and interesting, and wishes to see how things will unfold.
After creating his bride and producing three sons, Rathowm created many minor Devils from his own body, and they began to breed.
The hierarchy in the Devil Realm is quite rigid, and comparable to the hierarchy of the Gods.
Devils that hold the Ranks of One, Two and Three, are the Monarchs that govern over the different areas of the Devil Realm.
Devil Prince Prochine has a great castle in the Devil Realm, his father sleeps inside it.
The layout of the land seems to be like a basic society, they have taverns, towns and whorehouses, but it is unknown how they operate as a society.