Thanks again to my friend, Latent Kindly; he's very helpful and will gladly look at any work you want edited.

"Normal speech"
Thought

Sans tried to cover his head with his right arm, the force of the hit shattering the bone and throwing him against the wall. His shocked and pained expression exploding into a cloud of dust along with his body. The Demon waited for the dust to settle, before laughing maniacally, then walking to the exit.

The Demon was almost to the end of the hallway when the cloud of dust shifted. They, and the spirits, turned back in shock. An gauntleted hand burst from the large dust pile. Sans was alive.

He looked almost entirely different now. No longer did he look completely monstrous. He was shaped like a normal skeleton. Dust poured endlessly from his bones as if he was constantly crumbling. A large bone plate covered his chest. Ribs pointed in from the edges towards a large heart cracked into six equal pieces each coloured like a different human soul. His upper arms and knees were covered by a bone plates too. Around his waist was a bone striped tunic. Bone strips were laid vertically around his leg and forearm bones. His head was covered by a blaster skull helmet with a slit cut through between the eye sockets and the front of the maw removed. Blue and yellow flame blazed from his eyes out of the slit and his grin was crazed. A tattered cape blacker than the void billowed out behind him in a non-existent wind, occasionally catching on his pauldrons. His right arm was missing, snapped away from the top of the elbow down due to the hammer hit. In his one remaining arm, he wielded a wicked looking headsman axe carved from bone. Every so often his body shuddered and twitched without warning.

Dust spread everywhere as he slowly stood up, he eyed the edge of the sharp axe, before swinging it over his shoulder and staring straight at the shocked Demon.

"It's a beautiful world we live in." he said as he took a heavy step towards the kid. His voice glitching and void of all emotion but sorrow and regret.

"Friendships grow, love blooms. In this world...even in the darkest hours, you know...that there is always someone who still cares about you, who still believes in you. Papyrus was that person for me, for everyone...even you. But you killed him. You killed us all. Why? WHY? WHY KILL US?! WHY ERASE US?! ARE YOU TRULY SO CRUEL AS TO DANGLE OUR FREEDOM IN FRONT OF US BEFORE RIPPING IT AWAY!? ALL YOU HAD TO DO WAS LET US LIVE IN PEACE!! YOU WOULD HAVE BEEN LOVED! WE COULD HAVE BEEN A FAMILY!...How could you have done this? Were we truly not good enough for you. Were we ever really friends? Did you ever even want to be happy? Did you ever truly love us like we loved you?..." Sans whispered, before chuckling in a very disturbing way, tears flowing down his skull. The spirits were also crying. They felt the same as Sans.

"You've gone way past the line I once set. I've broken the same promise so many times. I've killed you so many times. I've died so many times. This has happened so many times. Heh heh heh. It's finally happened. I..H.A.V.E..T.R.U.L.Y..L.O.S.T..M.Y..M.I.N.D!" Sans tilted his head back and cackled madly before suddenly stopping mid laugh and staring right back at the Demon, tears filling his eye sockets. Papyrus closed his eye sockets as Undyne hugged him.

"The kingdom of Monsters lies in ruin. Dust saturates the air. Not a single other monster still lives. Your sins from all the resets weigh more than should be humanly possible to bear, but... you are no human. No human would do this. Kill us over and over again without mercy. I cannot allow you to live any longer. You had so many chances to change, to go back. You took none of them. If you truly wish to erase us then go ahead, but you shall have to go through me. I will not lose. I am the vengeance of all those you have slaughtered. I've had it with you. I'm going to shove your ass so far down your throat that when you crap, you sing beethoven. now then... L.E.T.'.S..S.E.E..H.O.W..D.E.T.E.R.M.I.N.E.D..Y.O.U..R.E.A.L.L.Y..A.R.E.!!!" he screamed, before launching himself at the Demon, axe raised.

THE EXECUTIONER STRIKES!

Sans did not bother to bring them back into the void. He swung the axe wildly at the Demon, who was forced to throw themselves out of the way. They were fighting a loosing battle, and Flowey knew it.

'Chara! You won't win alone! Let me and Frisk have some control! They can dodge, and I can use magic!' Flowey thought to them. They relented, and their eyes blazed red as the weed cape shuddered and came to life. Flowey used their determination to summon large bombs and he began throwing them wildly at the Executioner. Sans grimaced as he dodged, before the mad grin returned and he summoned a squad of blasters. They charged up quickly. The magic building up so much that they started cracking under the pressure. Sans grinned and they fired. With Frisk dodging the Amalgamate of evil was able to dodge the immensely powerful blasts. The blasts vaporised the tiles and pillars of the Judgement Hall around them. He then instinctively threw out his missing right arm. The stump glowed with the colours of the souls, before Justice shone bright and a yellow, futuristic looking blaster arm formed and released a blast of Soul energy. The blast was barely dodged by the Demon. He grinned as he summoned more blasters and a forest of bones. Flowey summoned countless bullets and bombs as Chara readied their hammer.

The two titans of power clashed with an eruption of power. Bones, bombs, and bullets collided in the air, exploding into dust and covering the already dusty opponents. They sped towards each other at an astonishing pace. The axe and hammer met with a loud bang, shattering any unbroken windows. The two fighters struggled to fight the strength of the other.

"Y'know what. I think that if you reset. If you never look at another weapon again. I may just forgive you. Heh heh heh. Why? D.O. .Y.O.U. .K.N.O.W. .H.O.W. .F.R.A.G.I.L.E. .R.E.A.L.I.T.Y. .I.S?! What would happen if one of the most important parts in this universe was erased? Would the universe fracture? Would we ever live again? .W.O.U.L.D. .I.T. .B.E. .W.O.R.T.H. .T.H.E. .R.E.G.R.E.T?!? What effect has me absorbing the souls had on this reality? .W.H.A.T. .W.O.U.L.D. .H.A.P.P.E.N. .I.F. .I. .D.I.E.D??" Sans mused, before kicking the Demon away.

Sans prepared another assault. Blasters fired and bones flew everywhere. He pointed his arm cannon at the Demon. It turned cyan, before unleashing a hail of light blue missiles. Frisk tried to dodge them, but as soon as they moved the missiles changed direction and flew straight at them, blasting them into a pillar.

"C'mon kid. Just give up. I ain't giving up this time, and I've got more than enough energy to outlast you. I could keep fighting for days. But, uh, .Y.O.U. .W.I.L.L. .N.O.T. .L.I.V.E. .T.H.A.T. .L.O.N.G... If I kill you. You won't be able to reset. You won't be able to load. You die now. .Y.O.U. .D.I.E. .F.O.R.E.V.E.R!" Sans yelled, swinging at the Demon.

The Demon leapt to the side, the deadly Axe barely missing their head. The arm cannon turned orange, and a blast of flame sent the Demon sprawling. Sans grinned as they threw more bullets at him. He spun the axe rapidly, before swiping it up into the air. A large barrier formed where he had slashed, and the bullets harmlessly impacted it. He shoved the axe forward and the barrier shot forward, exploding on impact with the Demon and sending them flying into a pillar. They got up, and looked around. He was gone. They stepped towards the exit, but jumped back to dodge the axe flying at their head. Sans stepped out from near the exit twirling the axe. He suddenly slammed it on the floor. The pillars shuddered violently before exploding into dust. The entry way door shuddered before collapsing. The ceiling was trembling and parts were beginning to fall. The arm cannon turned orange and it began to belch out fire towards the Demon. Sans cackled madly as the collapsing hallway was covered in fire. "You know. I always did hate this place!" Sans cackled. He loved the look of horror on that Demon's face.

He summoned a pair of blasters, they looked as cracked as he did. One charged an orange beam, the other charged blue. They began rotating in place together around an invisible motor, spinning faster and faster. As they fired, the blasts combined into a spiraling blast of blue and orange that filled the hall. The Demon was forced to rapidly change their momentum, but even they could not dodge the seamless transition between colours and were blasted into the floor.

Sans charged towards them, stomping an armoured boot into their chest before rapidly swinging the axe at them, heavy swings denting their armour. They had used a dozen more DT vials in just a few minutes, and most of their body was turned into the dark substance. As Sans continued mutilating the armour, The Demon felt the urge to throw their arms out. They did, and the black substance shot out like water from a firehose. Sans' eyes widened in shock as the torrent of substance threw him against the ceiling. The Demon grinned, before throwing their arm down. The substance followed, and threw Sans against the far wall with enough force to turn him to dust. Chara began cackling, but was cut off by a low chuckle. The dust flew into the air as if caught by a tornado and began clumping together.
"Nice throw. M.Y..T.U.R.N.!" Sans yelled gleefully as his body was completed.

Chara snarled, before running at him. . Sans grinned, and spun around, throwing a yellow cannon arm out as he crouched down. Chara was unable to stop in time, and soon the charging cannon was roughly shoved into their stomach. Sans' grin widened even further.

"See ya!" He sung, as the cannon finished charging. The Demon was thrown back as the cannon began rapidly firing blasts of yellow magic. Sans kept firing until the Demon was embedded in the far wall.

Chara tumbled out of the wall, catching themselves before they could fall. They shook their head, then caught sight of Sans. The Demon yelled as they rushed at Sans, prepared to swing the hammer down onto his head. Sans grinned and spun around, wielding the axe. As Chara swung down he used his momentum to cut the hammer head clean off. As the head came clean off, he slammed an elbow into their faceplate and back kicked them into a wall. A sickening crack was heard and they slumped forward. Sans went to cut their head off, but they suddenly sprang up and stabbed the hammer handle into his helmet. Sans staggered back, grasping the handle poking out of his eye socket, and fell against the opposite wall. A piece of the roof fell and trapped him against the wall. The Demon giggled madly and skipped over as they unsheathed the real knife. They went to slash at his head, when the noise of a blaster powering up made them fling themselves backwards before the purple blast vaporized the air where they were standing. They gritted their teeth and looked around to see who had interrupted them.
"Not another one!" They groaned.

An older skeleton wearing a lab coat had reopened the entry and walked in before sealing it off again, a pair of large and evil looking blasters hovering behind him. He muttered some kind of weird language under his breath before clearing his throat.
"Yes. Another one. Now then. Give up." He said.
"NO WAY!" They replied as they brandished the knife at him. He chuckled darkly.
"You have no choice. I have destroyed all the save points. If you die. You reset. So give up. OR I MAKE YOU." He threatened. The Demon growled dangerously before engaging him in battle.
"Pity. I had hoped you were smarter than this." He sighed before rolling up his sleeves. Just like Sans his eyes shone with a purple flame but his arms were engulfed by the same fire.
He swung his arms up, and large bones erupted out of the floor and hit the ceiling. The rumbling stopped as the ceiling was held in place.

Gaster swung his fists down, and the blasters opened their jaws, charging their lasers. Gaster thrusted his arms out as he opened his fists, and the blasters shuddered as they released their powerful blasts. The Demon rolled to side, and began to run towards him as they dodged the direction changing blasts.

Numerous bones tried to imped their path, but Gaster was already exhausted after destroying the save points. The Demon cackled in triumph as they jumped at the doctor, the momentum of their swing sending him flying.

Sans had just gotten the rubble off, when Gaster hit the wall beside him.
"GASTER NO!" He shouted, rushing over to embrace the monster.
"It… it is okay Sans...I will return, reconstitute...You will defeat this enemy, my son…" Gaster choked out, before fading out in his son's arms. Sans' body shuddered as he sobbed, before he froze and looked back. His eyes were a pure, blinding yellow. He slowly stood up.
"YOU! YOU WILL PAY FOR THIS!" Sans vowed, his words boomed out as if there were countless voices speaking with him.

The Demon was going to lose, and they all knew it. So Frisk ran… or at least they tried to.

"OH NO YOU DON'T!" He yelled, as the exit door collapsed as a powerful blast destroyed the supports. The Demon gulped as they turned around, shivering as they gazed at Sans. His axe glowed a blinding yellow, and seemed to shudder in and out of existence. They shook their head, before charging him, magical attacks collided in the air around them as they swung the knife up at his sternum. Like lighting, his axe shot through the air as he slid to the right, the Demon stumbling past him. They cried out in pain as they were forced to experience every time that had killed Alphys and Asgore from their perspective in a single moment.

They lurched in terror, attempting to swing the knife at Sans, but their arm would not obey. Their eyes widened in fear as they glanced upon their arm as it began shuddering in and out of existence. They turned back to Sans as another swing of the axe sliced into their other arm, forcing them roughly against a wall. They experienced Undyne and Toriel next, struggling to hold onto their Determination.
Another swing, and their legs were unresponsive. As they fell to their knees, they experienced Papyrus' death at their hands. Their body shuddered weakly as they tried to move their limbs. The Demon tilted their head up and glared hatefully at Sans.

Sans sighed, leaning on his axe as he gazed down sadly at the struggling Demon.
"You know… I really didn't want it to come to this... I wish there was another way, but there is no chance at redemption for you. We had so much fun together… Remember? Heh. Remember the time I stacked 30 'dogs on your head? Remember how happy Pap was to get that car? Remember when we had that housewarming party on the surface and Undyne tried to bake a cake? Heh…" Sans grinned, before frowning again. They were still struggling.
"I guess not... Welp… Goodbye… Forever." Sans sighed, as he hefted the axe above his head. The Demon's eyes widened in fear as he brought the axe down on their head. With a wet crunching noise.

Their body shuddered violently as their mind was slowly erased. All of their memories and feelings slipped away like water through their fingers. Sans waited for them to stop struggling before pulling out the axe with a chilling squelch. Their body flopped onto the ground lifelessly. Sans gritted his teeth as he lifted up his glowing white arm cannon.
"I'm sorry Tori." He murmured, closing his sockets. A single tear flowed from his closed eye socket as the cannon fired. The body disintegrated as the beam touched it, every last trace of it. Their now colourless soul floated weakly before shattering into millions of pieces. The world reset…

Sans shuddered awake, his eyes snapping open with a glaring light, before he calmed down.
"SANS! GET UP LAZYBONES! YOU WILL BE LATE FOR WORK! HONESTLY, I DON'T KNOW WHY THE KING CHOSE YOU TO BE THE ROYAL SCIENTIST WHEN YOU CANNOT EVEN GET OUT OF BED AT THE RIGHT TIME!" Papyrus yelled from downstairs. Sans blinked in confusion before 'new' memories came flooding in. He remembered being his father's assistant. He remembered being attacked by a strong glove wearing human. He remembered the experimental testing that saved his life, but changed his body. Wait… his body? How much had the universe changed? Sans sat up and glanced at himself. He was about as tall as he was in the last timeline, small traces of bone plating could be seen. His arm was still there though, That was a plus. He felt less tired than he was before, he felt like he could actually fight for longer than 10 minutes without falling asleep. Checking his stats further raised his mood, 100HP, 10ATK, 10DEF? He felt better than he had ever felt. Glimpsing at his clothing showed he was still the same personality wise, chuckling at the atom trusting joke on his T-shirt. Still grinning, He got up, got dressed, and headed to his 'new' job.

3 days later

Sans had to admit, this version of reality was great. A dream come true, he dared to think. He had a well paying job as Royal Scientist, with Alphys being Royal Engineer. His stats were above 'don't trip on the rug' level. Papyrus could actually cook edible food! But best of all, almost everything else was the same. He still did stand up comedy at MTT's hotel. He still had his powers. His friends and family were all the same. He still shared jokes with Tori. Pity the weed was still a thing. Gaster was still in the void too. Almost nothing could bring someone back from erasure. But aside from those changes everything was normal.

1 week later

Sans was on his way back from updating the Snowdin cameras, when he noticed the ruins door was slightly open. He turned to shrug it off, but then he noticed footprints. Small footprints. He followed the trail to the bridge and spotted a small human child holding onto the bars (he had upgraded the bars a few days ago). He walked slowly up behind them, right hand clenching around the familiar object in his pocket. The human froze as they heard his sneakers crunching against the ice.
"HUMAN. Don't you know how to greet a new pal? Turn around, and shake my hand." Sans said, pulling his hand out of his pocket. The kid slowly turned and grasped his hand. A loud farting noise echoed throughout the forest. The kid immediately relaxed and started giggling as he looked them over. No EXP, no LV, not even a single bit of dust.

"heh. the old whoopee cushion in the hand trick. it's always funny." He said. Time to go to work...

So. It's over... Well, kinda... This chapter is not edited right now, so it might change. Even then, I may write a continuation of the next human's journey through the underground. I just haven't really found the motivation or time to do much writing lately. But I will still write things here. I will eventually get to the fallout/undertale stories, eventually, but I have other ideas I want to explore with "DEFENDERTALE" My continuation of ideas with my O.C. from "Glitch", and a veritable flood of other O.C. characters as they defend a multiversal cluster from a seemingly infinite enemy. So yeah, I have stuff planned. But I figured, that right before an English SAC, I might as well just upload this. So yeah, thanks for all of the comments and I hope you enjoyed reading this.