The dead monsters will talk like this
Do not play music yet

sans was waiting for frisk to arrive, while also thinking about his brother.

Meanwhile in the background papyrus, Undyne, Alphys, Toriel, Mettaton, And everyone else frisk had been killed were watching sans fight the human.

"Sans can't do this" papyrus said quietly, making Undyne and the others look towards him "sans can't use magic" papyrus continued making more monsters turn there heads his way "WHAT DO YA MEAN HE CAN'T USE MAGIC?!" Undyne yelled "every monster can use magic if they have-" Undyne continued until she realized "sans doesn't have any determination" Alphys finally got enough courage to talk "b-but we m-monsters live o-off of d-determination"

"Let's just get to the point" sans started says, making all the monsters turn there heads to sans, seeing the human has arrived "it's a beautiful day outside. birds at singing, flowers are blooming" he continued making papyrus more and more anxious for his older brother "on days like these...kids like you...SHOULD BE BURNING IN HELL" sans finished in a dark voice, making his white eyes disappear and frisk's soul turns blue like sans's glowing eye and sans rammed frisk into the wall.

"I didn't know sans can use the blue attack!" Papyrus shrieked "let alone any magic!"

🎶 Play music now 🎶

Then bones appeared and frisk tried to dodge them. Soon after giant skeleton like monster heads appeared and as sans clenched his fist they shot a laser that frisk dodged. More of them came after that

"that is a-a gaster b-blaster" alphys said looking at undyne and papyrus "I've n-never seen one that powerful looking before t-though" she continued "who ever knew that our lovable and lazy skeleton could be so terrifying" grillby said with slight horror in his voice as he saw blue smoke coming from his right eye

Walls of bones came and frisk jumped them all.
Frisk started levitating and sans moved his arm side to side frisk following his arm, ramming into the walls and every time they touched a wall bones appeared and some went through her stomach. She fell to the floor and the knife when flying across the room almost hitting sans but he quickly dodged it

"Sans lies to me" papyrus said sadly as tears started to fall down his bony face. Undyne looked at sans and she swore she saw something red inside his jacket pocket

soon after more gaster blasters appeared and surrounded frisk. she held her had over her mouth just before she coughed up blood. sans was preparing to shoot but hesitated when he saw her crying with blood dripping from her forehead. but once she wiped her eyes showing everyone her blood red eyes, he then automatically clenched his hand in a ball making all the gaster blasters shoot. She ran in circles until they were all gone, then she ran behind sans got her knife and went right up behind sans holding the knife in her left hand.

She kept on trying to hit sans but he kept on dodging her knife she continued this until she fell to her knees, but this time she dropped the knife and tears welled up in her eyes and she cried.

"Stop it!" she cried, "I'm sorry! I don't wanna do this anymore! Don't make me do this!" she begged, "I'm sorry!"

Her begs made all the monsters feel a bit of pity but papyrus knew his brother and he was not going to let her get away with killing all the monsters in the underground

"You think sorry fixes anything?!" Sans yelled, outraged as he reached his left hand out and pinned her with his magic and drew nearer, "does it bring any of them back? Shyren? Undyne? My brother? Alphys? Mettaton? Grillby?" he flung her against a wall and felt sick satisfaction in hearing her cry of pain, the first she'd uttered since fighting him, "What about any of the other creatures you killed? The ones you spent hours seeking out? Do you think apologies mean anything to them?" he flung her against another wall as hard as he could and watched as she fell to the floor, cracks in the wall where she hit, "the monsters you need to apologize to can't hear you! They'll never hear you because they're dead! You killed them!"

All the monsters are keeping their eyes on the grieving skeleton, who was shaking with tears

"I-I know!" the little girl struggled to her feet but failed, "I know!" she sobbed, "I know, I know, I know!" she screamed.

"You knew exactly what you were doing, and you had every opportunity to stop and you threw it in their faces! You went back to fight them! You spent hours seeking out every monster you could to slaughter them in the most brutal way possible! You systematically struck each and every one of them down, and when you failed?" he flung her against another wall, then the ceiling, then down on to the floor in front of him, "You went back in time and fought them again." He hissed lowly, "You learned their moves so you could take them down and you killed them!" he threw her away against a wall so hard it shook.

"Sans might be taking this a tad to far" toriel says as she noticed the human "no if anything he is not doing enough" Undyne says

He walked over to her, eye burning as brightly as his fury, "and now that it's become hard-now that it's become too difficult for you to handle-now you want to quit? Now you want to apologize? Now you expect mercy when you couldn't be bothered to show even a scrap of it yourself?!" he raised his hand to finally deliver the decisive blow. He knew she would be back after, but he was done with this, for now anyway.

"NO!" the girl sobbed, "I didn't want to! I didn't want to kill them! I didn't want to hurt them! I didn't want to hurt anyone!"

All the monsters turned their attention to frisk. "She didn't want to hurt us?" Papyrus says with a bit of joy in his voice "but that still doesn't change the fact that she murdered half the underground" Undyne says, crossing her arms

"Well it's too late for regrets" sans says in a cold voice and makes a bone cage around her and he watches as chara takes control of frisk again

"Good luck getting out of that kid" sans said as his eyelids start to close. After a few minutes sans is fast asleep.

"Now is not the best time for sans to sleep" grillby says in a annoyed tone

chara reaches out for the knife and breaks the bone cage. She takes a few steps back and then charges at sans full speed but he dodged it...until the sound of drips echoed across the empty hall.

"SANS!!!" Papyrus shrieked as he saw the blood dripping down his brothers chest "he-he lost" Alphys says in such a small voice that everyone barely heard it

Sans looks down at his chest and sees the blood dripping from a giant wound going across his entire chest. Some blood comes dripping out of his mouth.

He stumbles back a bit until he sits down, back against a pillar. He slowly pulls his hand out of his pocket to reveal papyrus's scarf around his hand.

He starts to sniffle, trying not to cry but the tears start to come down his face

"I-I fail-led e-everyone" he cried as his skull starts to dust away

"SANS!" Papyrus yells falling to the floor, tears rushing down his skull "papyrus" grillby says as he puts his hand on papyrus's shoulder "sans will be with us now"

"papyrus. Undyne. Alphys. Mettatton. Grillby. Toriel. Asgore." Sans crys "I'm sorry" he says as he holds the scarf tightly to his chest and the rest of his body dusts away into thin air.

Meanwhile frisk is crying her eyes out. After 2 minutes she moves the knife to right above her soul and was about to plunge it into her chest but stopped.

She turned.

And left.

Leaving a pile of dust sitting peaceful on the ground but Papyrus's eyesockets widened as a little white heart materialized, floating above the dust. The three stared at it and its soft white light.

"Sans was a boss monster?" Undyne was the first to say their thoughts. A very, very trivial thought compared to the bigger 'Sans was dead'.

A crack split through the middle of Sans's soul and then split down in half. Papyrus wouldn't avert his eyesockets from what was left of his brother. He'd stare until the soul broke entirely. Alphys joined him in his vigil, still clinging to Undyne and with tears still in her eyes. Undyne, in a rather large contrast, glared at the little soul, as if saying 'don't you fucking dare leave me with a sobbing Papyrus'.

They waited for the soul to break.

And waited.

And waited.

And waited.

It didn't.

Needless to say, they were all very confused. Sans just died, he turned out to be a boss monster, and now his soul won't break. Perhaps that was a good thing, but then again maybe it wasn't. Was Sans still conscious?

Papyrus open his jaw to ask when suddenly the heart erupted into blue flames. All the monsters gasped at the sight of sans's soul engulfed in flames, startled by the spontaneity of this as the broken soul glowed blue, magical flames licking at the air. A small white bone materialized out of nowhere and crashed the end against the corner of the bookshelf, breaking one end into a point. The little bone then dug into one side of the broken heart and pulled out, tugging behind what looked like a blue thread. Then it dug into the other half, and then poked out, and then the other half. In, out, in, out.

Alphys, Undyne, and Papyrus stared, watching the needle as it worked. When the BoNeedle made it the entirety to the bottom, it began to tug the thread tighter, bringing Sans's soul back together. Sans's soul put itself back together. And once it did, the dust that was his body swirled into a storm, glowing blue as Sans reformed. From his soul, strings reached out and recollected the dust and dragging his clothes around the cloud, the glow died down and Sans plopped onto the floor, rubbing his skull. "Jeez that hurt" he said as he got to his feet and looked around spotting the blood splattered everywhere. "Looks like I'm going back to human hunting" sans said to no one and he walk through the same hallway frisk left in, leaving the monsters in the empty hall