Not entirely happy with how this came out, but I hope you like it! I've been wondering how did Sans become the judge, and...

Judge

The hole building was dark; the power supply had suddenly shut down. Alphys didn't like it. Sans agreed with her and evacuated the building just in case, but when the tremors started, the skeleton ran back into it. Gaster hadn't gotten out.

He didn't hear or care about Alphys' worried shouts.

Sans ran as fast as he could. He had a bad feeling about all that. Experimenting with timelines was dangerous and he knew it. He should have stopped Gaster, tell him to focus more on improving the Core, trying to find a way to break the barrier or starting those experiments about the human Souls to create the last one they needed or, at least, a substitute.

But they were talking about controlling time. Endless hours to make every discovery. Endless hours to rest from work. Endless hours to enjoy, work, and live. Rewinding time at will and going into the future. Even (even) going back in time so far they could change the war. They could be the ones who won and banish the humans into Underground. Or stop the war completely. Or avoid it.

Too tempting to let it pass.

And maybe that had been their worst mistake. Their worst sin.

Sans arrived into the room where Gaster was when the tremors were so strong they threatened to collapse the building. The scientist didn't turn after hearing his shouts. He didn't move when he heard Sans' footsteps getting closer. He didn't answer when the other skeleton said that they had to get out of there now, before the whole place fell on them. Gaster simply kept staring and what he had in front of him. The machine they had worked on was turned on, and a hole floated just in front of it. It had no color, and you could see nothing through it. It didn't move and it didn't change shape. And Sans suddenly knew it shouldn't exist.

"The experiment failed." Gaster whispered, in a slow and monotone voice.

Sans managed to stop looking at the terrifying abyss and stared at him. Gaster turned his head slightly and took San's hand. He put a syringe with a red liquid on it. Sans recognized it. It was the first and only successful extraction of Determination from the human souls. The Extractor was still far from done, and that shot was extremely imperfect. Gaster had calculated than less than 20% of it was pure Determination. They couldn't experiment on in and expect any result, and it was useless. But they had preserved it because it was the proof that it was possible to extract Determination from human souls. Maybe with the new machine Gaster had made blueprints of.

"Live." Gaster said, making Sans snap out his thoughts.

At that moment, there was another earthquake. It was so strong the ceiling started collapsing, blocking the entrance. Then, Gaster touched the hole. They both disappear in an instant with a shattering sound that roared through the hole building. Something had broken into endless pieces, scattering everywhere and anywhere at the same time.

And Sans found himself suddenly alone in a room with a ceiling that could fall down and kill him at any moment. The syringe weighted and he was sweating. There was nowhere to escape and he was running out of time and didn't have time to think. The syringe was still in his hand, and he looked at it. If Gaster had given that to him, it was because it could help him escape alive (or so he wanted to believe), but there was nothing to inject the Determination on except himself, and he was made of bones. There was no way his body could simply absorb it without performing himself, and he didn't believe the needle was strong enough to do that. There was only one other option. Another part of the ceiling collapsed near where he was standing.

He acted without thinking, knowing rationality would make him more scared about it and his mind would try to stop himself.

Later, he wouldn't remember the sudden power he felt on his body. He wouldn't remember the way his blue magic stopped several pieces of the building in mid air like they were feathers. He wouldn't remember the skulls that appeared out of nowhere, opening their jaws and shooting a powerful blast that destroyed everything that was on him.

He would only remember the burning pain in his left eye while he sunk the needle right into the light of the eye socket. He only remembered his own shouting through the collapsing room.

Alphys had been walking around the room several hours. Her legs hurt, but her worry didn't let her sit still. A quiet groan surprised her, and she hurried towards the bed.

"Sans! Oh, my God! You're awake! H-how do you feel?" she whispered, almost crying of relieve.

The skeleton managed to look at her, confused. His body felt exhausted and his head hurt terribly. He didn't seem to be able to focus, and slowly raised one arm to his head. He felt a bandage on his head and then realized only one of his eyes had open. The left one was covered.

"a-alphys? what...? why are you crying?" Sans muttered, changing the question when the reptile suddenly started crying.

"I'm fine, I-I'm fine." she reassured him "I-it's just... you've been unconscious f-for five days a-a-and... we didn't know if you would... if you would..." Alphys didn't manage to finish the sentence, but her relieve was almost tangible.

Sans sit immediately after hearing that, before she could stop him. The movement made him feel dizzy, and he feared for one second he would lose consciousness again.

"papyrus." he said "he must be worried. i have to go home!"

"Don't worry!" Alphys stopped him before he would get out of bed "We called him pretending to be your college teachers and told him you would be living there because you suddenly had a lot of work and exams. And that he couldn't see you because they were really important and you needed to concentrate. He told us to wish you luck." she said, with a warm smile "And that you didn't need to worry about him, just concentrate on your work."

Sans looked at her, tired, and then sighted, feeling guilty for the lies Papyrus had been told. But he wasn't worried about his brother taking care of himself. Heck, he was more capable of taking care of everything around the house than Sans. He was the one that would clean everything, make sure they didn't run out of food and call whoever they needed if any repair had to be done. If Sans lived alone he was sure the house would have collapsed on himself out of spite in less than a week.

He still asked Alphys for a phone to call him. It had been five days and Papyrus had to be worried.

"Howdy, Sans!" King Asgore greeted the skeleton, who had just called Papyrus. After spending a week recovering from his wounds, he would finally be able to go home. "I have been told you are feeling a lot better. Is that true?" he asked with a soft smile.

"yeah, your majesty. i'm fine now." Sans answered, not surprised by his visit. The King had frequently gone to check everything was developing well and his health was improving. "paps told me he would buy something special to celebrate that i finished my exams and that i'm going back."

"That's wonderful." the King smiled. Then he stood there, in a tense silence. Sans looked at him, confused, and Asgore stopped smiling. "But there is something I need to talk to you about before you leave." His voice tone revealed he wouldn't take a 'no' for an answer. Sans had a bad feeling, but he had no idea what he would want to talk about, and no real reason to refuse, so he just followed him out of the room.

They walked in silence through various corridors, slowly but steady. They finally reached a golden corridor. Little light reached the Underworld, but the shiny metal made it the brightest place Sans had seen in his life. At least, when it come to natural light. Sans couldn't stop looking at it.

"Do you know what this place is, Sans?" the King suddenly asked.

The skeleton didn't. They had taken a path that was always closed for everyone. As far as Sans knew, no one had ever entered that place.

"Long time ago, after the war had ended, we were sealed in the Underground. But some humans weren't contented with that." Asgore added before Sans could stop him to say that yes, he did, in fact, know history "They knew that a single human soul was as strong as almost all monster souls together, and some of them would cross the barrier with the only purpose of killing monsters. Once they got bored, they would kill a Boss Monster and leave. I do not know whether they did this out of hate, bloodlust or simple boredom, but it was a dark time in our history. We almost lost all hope."

The King paused and Sans didn't know what to say. What the hell was that and how exactly did that relate to Sans, anyway? Humans seemed to have forgotten about the Underground a long time ago, and only lost children had found themselves there.

"Since we didn't know humans' intentions, but they had to be stopped somehow, it was decided that one monster would take care of the situation. The monster had to be strong or skilled, in case they had to engage battle, and they had to be stealthy and able to move fast between places. The monster would observe them, follow them. And then, they would decide whether that human deserved to live or whether they were too dangerous for monsters to be left free. Three more humans came. Two of them died in the Underground, one of them in this same corridor where they were judged, but we didn't have the technology to preserve human souls for a long time and they shattered. The third one managed to escap, but no human came back for centuries. Until my son arrived."

King Asgore paused, but Sans already knew what he was asking him.

"why?" he asked the King before he could keep talking "i'm not strong nor skilled. i'm just a scientist who can't even clean his room." he refrained himself from saying that he didn't have a place to work at anymore or that his partner was gone. While he was recovering he had discovered no one seemed to remember Gaster. It was like he had never existed.

"I visited you while you were unconscious. I saw them." Asgore answered.

Sans stared at him, with a knot in his stomach. He already had discovered the weird things that were happening to him since the day of the accident. Suddenly appearing in another room. Pulses of blue magic scattering and breaking everything around him. Creepy skulls looking at everything intently. Only one of them had actually shot some kind of laser beam, destroying the room. That was the third time Sans had to change the place where he was resting, anyway. At least no one had gotten hurt.

He was starting to control whatever that sudden magic he felt was, but it still made him uneasy. It had appeared right after Gaster had stopped existing, and the possibility of himself disappearing didn't make him happy.

"If the Judge decides that those humans are not here because of cruelty or maliciousness, they shall meet me for a chance to go back to the surface. But you have every right to reject this offer, Sans. I'll understand." the King gave him a sad smile.

you're a coward. Sans thought before he could stop himself. That wasn't a job offer. He was being asked to either kill the humans or let them kill his king.

"If you accept, your brother and you will move to Snowdin." Asgore's voice sounded tired. Everything had changed when his children died and his wife left him.

Sans left the room right after that.

Telling Papyrus about the "sentry job" was something Sans regretted almost instantly. His brother had become extremely excited when he knew Sans would have such an important job (capture humans, nothing less!) he tried to convince him to accept it since Sans had suddenly lost interest on studying for his dentist degree. Papyrus thought it was because Sans had failed the exams, and that changing their environment could be good for him.

Sans finally accepted because he didn't have any future plans and well, he thought six human souls had already been caught. we only need one more. it can't be that bad.

When the voice of the door asked him to protect humans, and a scared kid laughed at his whoopee cushion, Sans wasn't so sure that becoming the Judge had been a good idea anymore.

When the kid went out of the door covered in dust and simply stared at him like he was prey, Sans wondered whether breaking his promise right there would be that bad.