There are those, learned and respected scholars all, who will tell you that the first Great War was fought between the lizard folk and the serpent folk who were the gods of the Yuan-ti. They spend much of their time combating their minority colleagues who insist, no, the first Great War was fought between the aarakocra and the thri-kreen. Neither side does much more than emit scoffing laughs (laced with a disturbing sense of directionless embarrassment) when the wild-eyed and unkempt seers speak of even older things.
In the mind’s eye of poets and madmen dance a race of children. These children dwell in the happy spring of our world, cavorting and making as they dance and sing. Since this world is all they know, they name their mother the All-mother, and they name our world All Creation. And they shape the beginnings of our world with all the love, curiosity, enthusiasm, and vicious tyranny of children in the nursery.
And then something from Outside came. To say it invaded is to imply agency and choice. To say it tumbled implies it was pushed or tripped. So lets just say it came.
It was utterly inimical to the games and songs and dances of the Children. Their voices quavered in its presence, their gardens wilted, their games went all sideways. They couldn’t even speak with it, and its utterly alien ways went beyond uncouth, beyond creepy. It was utterly abominable.
And, by its very presence, it warped things. It didn’t so much spawn as twist things already in the nursery world and remake them in its image, to fit its idea of what nursery ought to be. It was, to the minds of the Children, the ultimate theft of their toys.
And this they could not stand. They made war against it. They tried to burn it with fire and freeze it with ice. They sang at it and they threw stones at it. But everything that touched it warped and became part if its corruption of the world.
Even worse, its corruption was infectious. The Children built an army, a massive collection of soldiers, all unified in purpose and of one single mind. At the first touch of it, a wave of instability passed through the entire army. The Children were forced to destroy their soldiers before they became a tool of their enemy.
Their next army was far more clever. Each was not just a unique individual, but misanthropic as well. Each individual specimen was a singular army in itself, armed with every clever weapon the Children could devise, it eschewed the company of all others, and most especially those like itself. Thus, should one fall to corruption, the others would be unmoved.
We call the by-blows of these warriors of the Children beholders. And, in the end, they murdered their creators, whom they considered just as repellent and horrible as the enemy they were created to destroy.
The beholders also won the war against it. They created an army of their own, an army of raw chemical hunger that sought only to dissolve everything, rendering it into fuel to grow itself. We know the remnants of this army as the various slimes, oozes, puddings, and gelatins that still lurk in the dark places.
So it was defeated, but the corruption it left behind was not undone. For, you see, the Children were right about their mother. She was also the mother of it. And it was a daydream, a fantasy of children-that-might-have-been, and its very presence implied into being the nursery-that-could-have-been-if-only…
And you can still find those echoes of its existence in the aboleth, and in the twistings of our own mortal shells that we call the illithid, and in similar horrors that do not belong and are not right. And we rightly recoil in horror and destroy to the utmost these terrible children of the Mother Of Us All. For their continued existence whispers in our heart-of-hearts that Mother loves them more than us.