how does a primordial/yozi become a neverborn?
First you have to understand what the Primordials were.
Each of them was a world unto itself, a set of interconnected concepts and beliefs so powerful that they branded their meanings onto the Chaos That Was and forever changed the universe
just by being born. If you've heard mention of Infernal Exalts or Yozis having a capital-M Mythos, it's referring to this quality of theirs: each of them cries out their identity to the cosmos and forces it to acknowledge them, forever engulfed in total comprehension of their self-concept. Raksha play at being able to make up their own rules, but the Primordials can actually do it - if only within their personal themes*.
This power is why the Primordials have a soul hierarchy; their inner thoughts and opinions are so metaphysically potent that they take solid form just to properly express themselves. The unity and infinitely complex interactions of each Primordial's spiritual anatomy completed the tapestry of their Mythos, and at the heart of it all lay the Fetich, central pillar of the Primordial's sense of identity. The system of Virtues and Intimacies that define the Primordials' creations is essentially a crude facsimile of this, with Motivation standing in for a fetich.
Yet where gods and men are fickle, the Primordials' hearts were eternally steadfast**. Love and fear, adoration and disgust, whatever the Primordials were - however unsightly, however painful - they
were, always and forever. Nothing but the greatest of efforts could change even a fragment of their mind, and even then their Mythos would seamlessly shift to accommodate the new perspective. Most of all, none of a Primordials' souls could ever truly die, because the remains would scatter into the greater chorus of their Mythos and eventually be reborn. You could no more destroy The White Ram than you could strangle the idea of seeing, no more cease the flow of Lilike than you could make someone's sadness into a hat***. It was impossible, utterly impossible.
So then Autochthon, in his fury and grief, made weapons to surpass possibility, and the Primordials wept to see what came of it.
You see, that's what it means to kill a Primordial's subordinate souls - as it disintegrates, the very idea that it represented within the Primordial's greater self is incinerated, scourged from that Primordial's Mythos forevermore. The closest mortal analogues are intense psychological trauma and crippling brain damage, neither of which properly capture the sheer horror and violation of what was done in the Incarnate Rebellion. The Solars almost literally tore the Primordials' loves and beliefs from their heads and smashed them into bloody shrapnel on the ground.
Those who were "lucky" only lost a few of their souls, but became the Yozis, forever diminished and forever suffering at the ragged holes left behind in their Mythoi and the knowledge that they would never, ever be whole again - or even be able to truly understand what they had lost.
A few suffered a worse fate - the Exalted butchered their Fetiches, and without a defining thesis, those Primordials' Mythoi imploded on themselves, only to stabilize as inverted mockeries of what they had been.
The Lidless Eye That Sees, patron of perception and mindfulness, became Sachaverell, a creature locked forever inside its own mind,
dreaming instead of
seeing. Adrian, who represented pain and the passage of pain, who bore a billion scars and treasured each one as a mark of his connection to the world, was murdered to make Adorjan, who values nothing and treasures nothing, denying all connection to remain mindless, bodiless. Elloge, the Sphere of Speech, lives in worlds that resemble neither past nor present, and so defiles the Mythos of the being she was - He Who Bleeds the Written Word, supreme scholar and recorder of histories, cartographer of what Was and Is.
Such desecration is nothing compared to what befell their brothers and sisters who are now Neverborn, and Never-Dying. The Incarnae and their servants tore away at them until there was nothing left, burnt their Mythoi to ashes one piece at a time until they collapsed in upon themselves, unable to think or feel or do anything but blindly relive their own murders again and again and again, while the last echoes of consciousness desperately struggled to understand the impossibility of what had been done to them. A Primordial's Mythos defines its world, and by taking that away the Solars destroyed everything the Neverborn had ever believed in. Broken beyond recognition, beyond the ability to live, still the ruins of each Neverborn's Mythos screamed "I AM" to the cosmos, and so they could not die. They broke Creation in their fall, their corpses heaping pathetically at the bottom of the universe, and great sections of the world crumbled away and fell upon them, settling and mingling with the curdled Essence that spread out from their resting place like a pool of blood. The first became known as the Labyrinth, and the second is called the Underworld.
Does that answer your question?
* (So much of Creation's underpinnings - the dragonlines, the Loom of Fate, even the establishing of time and concrete distance - was just so the Primordials would be able to have a point of reference that wasn't either themselves or one of their siblings.)
** Except the Dragon's Shadow - sort of - but fuck him anyway. Also, Autochthon jailbroke himself out of this, and nearly died for his troubles, so great job there.
*** Don't even say a word, you raksha fucks.