Edit: Since some people has been having a lot of doubs, let me explain some things.
1. Fanfiction has a limit in the characters for the summary, the complete summary is this:
For ages, the Void godness has to sit and see how her multiverse was fill and filled by AUs of her Universe, Undertale, she hated them at first, but now all her hate fall in their creator, Fate's spawn, Ink.
For ages she has been incapabel to help those children of hers outside her Universe, the so-called Dark Sanses, the Guardian of Negativity, the God of Destruction and those under their protection, has to watch them beign hurt over and over again from the hands of their Universes or by the hand of the self preclaimed protectors of the Multiverse, the Star Sanses.
When Destiny comes to her with a plan to put them in a safe place, she inmediatly agrees.
Meanwhile, in another Multiverse, other Void God makes a deal with the ruler of Hallownest.
The Pale King would give him back ALL of his children and then he would help him against the Infection caused by the Moth godness.
So he is very confused (and a bit mad) when months later he can feel six more children enter to Hallownest without an explication.
2. I really dodn't expect no one to read this, giving the lack of Hollow Knight and Undertale Crossover in this page, so I don't really bother to much in explaining this thing here when I already post the same thing in AO3 and Wattpad.
3. This story use the Multiverse of Undertale, so characters from Underswap, Underfell, Ink and Error Sans are gonna appear (the last oneis one of the protagonist on the story)
4. This story is using the concept of "Forced God of Destruction" for Error, this concept was made by harrish6 in AO3/Wattpad
The only thing I own here is the idea of the story and the Void godness, nothing more, ok? ok
Once upon a time, a multiverse filled with void exist, one single universe could be observed in the infinite black, one universe that was created by the void, and the void was happy, that universe was hers, to protect, to take care of, and even when one of her children fell directly into her void, she didn't destroy him like usually would do with other things, no, he still exist and is capable of interact with the universe, even if he can't return to it.
Her universe and children weren't perfect, but she would never want them being like that, they lived, they love, hate, were in peace, fight in wars… but soon or later, died, and one part of her always hate that part of them, but was pacifier by the knowledge that their souls would return to her void once they died, and then they would be safe and sound in her arms.
(When the resets began and the souls stop come to her, she was mad to her insolent child, playing with the lives of their siblings, and causing pain to their brother, so she left one soul return to the world to tech them a lesson, her little Chara wouldn't let them to this so called "Genocide" without a cost).
(That didn't stop Frisk to do such terrible things to the Monsters, but their fear and rage each time Chara "ruins" their attempt to do a "Pacifist Route" was enough for now).
Then, something happens and another universe came to existence.
A universe that wasn't hers, that didn't bear from her void.
It was… a copy of her universe, everything seems similar to hers, the people, the monsters, the resets, everything, except that none of them feels like her children, everyone in her universe have a little of void in them, inside their souls, that is what made them her children, this universe don't have void in One. Single. Soul.
When the Gaster of this universe fell in her void, she didn't twice before rip apart his existence out of the multiverse.
She was so mad, who was the responsible of this aberration? Who dare to take the image of her children and take away they void?
She hated this new universe and whoever have created it, so then she looked for a weak point in it and destroy it, she took everyone in the universe and tore them apart in the moment they rub her void, even when one part of her feels bad (they look exactly like her precious children, how she could not feel bad?) a more vengeful part of her is satisfied, maybe this would teach to its creator to not mess with her multiverse, with her universe.
Of course, not long after that, she felt more universes come to existence.
Oh, she was capable of get rid of the copies of her universe, but the others? Or the one with the personalities of everyone swapped? Other with that horrible "Kill or be killed" law? (and she could almost feel her heart get a little warm to them for so horrible life situation).
She couldn't destroy them, and not for lack of attempts, there was something messing with her ability to enter into the codes of the universes, it far from her reach, she couldn't enter to this universe like she could in hers, and the codes of these universe were inside them, so, she was powerless to destroy Underswap and Underfell.
She hated them at first, but as time passed, she was able to tolerate them, her universe was not being harmed by their existence, she made sure of that by completely blocking the access to all those who did not have Void in their souls.
But where she didn't hate the other universes, she despised their creator.
Ink, God of Creation and self-proclaimed Protector of AU's, the son of Fate.
Apparently, Fate decided that her multiverse was "empty" and that her son was "doing her a favor" by fill her multiverse with more universes.
Void knows better, her multiverse was being use like the playground of Ink, no more no less.
Other three universes were created when something changes.
Aftertale, in her eyes, was just another copy of her AU, with the difference that were more genocides than in hers, then, the Sans of the AU do something and ends in the Save Screen, he was so close to the void that she could just kill him if she wants.
She doesn't do that.
That Sans was so scared, so angry and sad for being in the Save Screen, away from his family and friends, away from his brother, his only reason to keep going each day…
He called for help.
The Void responded to his call.
Aftertale! Sans, Geno, becomes the first child she have outside her universe, Aftertale still having a Sans, and soon it became obvious that Geno couldn't get out the Save Screen, so, when her void began to take place in his soul, he didn't have more options that let it happen.
His face the first time she talks to him was adorable.
The life in the multiverse was good for years after that, Fate's spawn still creating more and more, but her children were okay, Undertale was having more Pacifist routes after Chara took over Frisk body and Geno was having something with Reaper! Sans.
(She wasn't really sure about the relationship, but Geno was so happy so she couldn't really complain about it).
(When they fused their souls and became soulmates and, by extension, Reaper becomes one of hers, all her doubts disappear).
Then, one day Fate seems to decided that her happiness, the happiness of her son wasn't allowed in the multiverse.
One moment she could feel perfectly the soul of Geno along with the rest of her children in Undertale, to the other, she could feel one part of her void being ripped from her.
Her pain was soon taken away by the rage.
¡She couldn't find Geno anywhere! Not even the image of him like the other AUs, it was like he disappeared in the air.
He wasn't dead, if that were the case, not only her but Reaper could have sensed it, his soul would have merged with her void like all his siblings did before him, and the other god seems as confused as her when find the Save Screen empty, his soulmate nowhere to be found.
For years and years both of them search, in one point Reaper even ask for the help of Fate's spawn (and for Life herself, if he bring back her child, she would never say something against the Creator again).
But the time move forward and Geno don't appear, but… one day she discovers what happen to him.
She finally met Destiny; Fate's sister who seems hate as much as Void what she was doing to the multiverse. The goodness was very vocal against the actions of her sister when she wasn't around, but she was as powerless as Void when comes to do something against Fate.
Destiny was the one who told her were was Geno, what was happening to him and what Fate has to do with that.
By the end of that she didn't knows what emotion rules over her, sadness or rage, the tears won't stop coming from her eight eyes.
Her poor child was being torture, transformed into something else in a place called the Anti-Void, completely out of her reach, and for what? Just because Fate refused to put a limit on the endless creation of her spawn?
The first time Error, the God of Destruction put a foot outside the Anti-Void, she cried once again. Even she still could feel her void in his soul, clearly marking him as her lost child, Geno, it became soon obvious that Error didn't remember being Geno, didn't remember her or Reaper.
And as the years go by, her hate towards Ink and Fate increased with each time Error was hurt, mental and physically, with each time his son tried to commit suicide, each time he asked for help just to be attacked by the blind judges of the multiverse.
But even in the dark times, she soon finds more children who need her protection.
When the Dreamtale universe was created, she sees an opportunity to give Error an ally, the guardian of the Negative Apples was also, without the knowledge of no one in the universe, the Guardian of the VOID Apples, those two being similar enough to fuse into one.
And when he ate the apples, incapable of take more of the hate of the villagers and the disinterest of his brother anymore? He was soon turn into hers, he was cover in liquid void and his soul now having void like the rest of her children.
The faces of the villagers before they were killed by him, and the face of his now ex-brother before he was turned into stone were priceless.
It takes some years before Nightmare was capable of hear her and trust in her, but once he does, he was on his way to help to the forced job of his brother, taking under his protection others like him, abandoned by their universes and alone in the world until he finds them.
They were soon find having void in their souls, children of hers even if they can't hear her at the beginning, they were hers and she loves them anyway.
The next time Error was destroying an AU, they were there to kill everyone there, making his job faster.
She wasn't looking the first time they meet, but she see the results, it went perfect, of course, Error was suspicious at first, believing that maybe it was just a trap or a cruel joke, but with the pass of the years he opens and comes to trust in his brothers (the void in his soul making this easier to do, it was natural for him to trust them).
Reaper was another story.
The loss of Geno, and the later knowledge of who Error was… it was a strong loss to him, the person who he loved the most was now lost, and didn't have the bravery to try anything with Error, he believed that wasn't fair to no one, even when his soul hurts by the tough of let him go.
Even the kind words and company of Void couldn't do nothing to comfort him, he throws himself to his work, everything to ignore the pain in his chest, just in countable occasions get off his way to help in the destruction of one or other AU.
Void couldn't really blame him for that, even if see one of her children being so depressed, there was nothing she could do respect to that.
And the time pass in the multiverse, Fate's spawn formed a team called the Star Sanses with Nightmare's ex-brother, Dream and the Sans from Underswap.
Then he made the Council of Sanses, whose objective was to stop the destruction of AUs by the hands of the now called Dark Sanses.
Many years later, Destiny comes to her with a plan to take they children away from Fate's hands.
Then that dammed ambush happens and was time to implement said plan arrives.