t was a typical UnderTale au, except sans was getting super tired of the resets that acured over and over again.
He wanted it all to end and to either get their happy end or get stuck in the underground with Frisk and chara gone.
He tried talking to Frisk, threatening Frisk, everything he could to get a forever happy end, it didn't work. So no matter how much he wanted to NOT do it, he tried to find a way to execute so to say, Frisk. Of course this mission seemed impossible as Frisks determination was strong as truck.

But seeing his family and friends die over and over, seeing them suffer made him hate humanity almost as much if even not more than Chara. So, no matter how much hope he had for changes, he snapped.
On a pacifist run, sans teleported Frisk straight to the judgement hall, confusing Frisk very much so.
She just took out her knife getting ready as she knew, was confident she'd win. But oh boy she was so wrong.

Sans revealed something that made Frisks blood freeze in her veins, Sans took out all six souls. Bravery, Justice, Kindness, Patience, integrity, perseverance and stared emotionlessly at Frisk.

With a smirk on his face he fused his soul with all the six souls, making him extra powerful. The sort off power that could beat determination, cut through it like a piece of cake.

He combined all the powers into a single gaster blaster, and literally dusting Frisk into oblivion. Frisk didn't have a chance, not a single chance was to be.

She died, died died. No reset would be done anymore, the world was back to normal. But not.

A bread crum like bit off soul from Frisk stayed floating in the air, sans took it combining his soul with it again. Surprisingly it still felt quite powerful.

After a while he realised he could free everyone, let all the beings of the underground live on the surface and be free!

But he did not want that.

Seeing the ruthlessness of humankind, he didn't want his family and friends, anyone to suffer the mindlessness of Humankind actions.

So he did what he thought was best.

He made another barrier.

Except this barrier, was made up off all six souls with an extra boost of determination. It could be deemed unbreakable. It had like literal six layers off magic and then after the original barrier. But it also came at a cost.

Since his soul was merged with the others he also,his soul was also made into a barrier, deeming him dead, or dormant in a form of magic so to say.

Everyone figures out immediately what happened, as after he done that the underground felt a huge gush off power throughout the entire underground.

That wasn't the only thing that happened. This magic made a change on some creatures magic and appearances.

Mostly skeletons.

From that day on, each skeleton that was born was born with one off the soul traits. Papyrus just got a boost of what was already seemingly a bit of human trait.

The barriers duplicated the population a bit and expanded the underground, magic poured over the underground like snow, making all six types of soul skeletons appear first then duplicates off other creatures too.

That was their life from their on, everyone loafed sans for making it impossible for them to escape the mountain as they never knew the full story behind it.

Few decades? Centuries? Later.
Den was born
He was born a determination skeleton, probably one of the most powerful kinds really, but that wasn't what made him stand out. What made him stand out was how terribly closely he represented the one they all hated and despised, Sans. Combining him with also the soul that brought the doom upon him he was shunned from society and looked down upon as the curse of the underground. Some feared him, some brave spat on him as if he was lower than dirt.

He was kicked out his house at the age of 5, well 4 and three quarters to be exact but still, he was young.

He first tried to fight back, but soon realized it was a bad idea as it made them hate and fear him even more. So he gave up on that.

The only friends he had growing up was one Bravery Skeleton named Joseph (determination skeletons were given human or so names, not font names, those were given to normal skeletons) and a dog named Steph.

Their parents didn't like that they were friends with him so for his safety they didn't hang out together much, but had monthly or so hangout days.

He was homeless, and poor and literally fought to survive through the hard times, pretty much everyone day.

And to be honest, out off everyone, he hates Sans most, he made his life this, this torture, he despised his view and didn't ever want to look in a mirror.

He was determined to one day, free himself from these chains off hate. It seemed impossible except from the one escape, death.

But his cowerdness, or stubbornness more like prevented him from doing so.
So he tries to find another way. He didn't know how but his determination made it so he, even though didn't show it, was determined to keep on fighting for his freedom.

This is poorly written as it was a quick type on my community tab and I just copy and pasted it here.
Hope you enjoyed reading anyway
Once I come up with an au name might make a quick book bout it maybe 🤔