Rugio Tenebris
Known to the living as "The Abyss" or "The Roar of Darkness"
The Abyss is situated at the bottom of the universe, flowing like an ocean bellow the countless worlds that exist, awaiting for those who inhabit to heed its call, and succumb to the pitch dark waves who desire to devour them.
The Abyss itself is a conscious being
Sinanima or "Demons", as they are known in the worlds above, are the personification of The Abyss. Once merely avatars with little distinction between themselves aside from their appearance, after eras in contact with the living worlds they've developed more unique personalities and even a similar societal system. Of course, the values of those above will matter little to the abyss.
Demons can be summoned through rituals writen down in grimoires and other ancient texts. The first texts mentioning the summoning of such beasts had no warnings attached and merely stated the wishes the summoner could be granted, like flies to the rot, those who took the risk of the unknown soon would learn the payment for their wish. It is believed that those rituals were first dropped by Sinanima, which would explain the lack of warnings, which started to be present in later texts.
Making a deal with the devil felt inconsequential in life, where death was all they had to offer, but now said death is their life, and they pay each day for their recklessness
The deal is flipped, and the masters become the servants. If deemed useful by their masters, they may be given tasks and even accompany them to the living world once more, if not.. well, to sate their master's cravings is more than expected