Please don't be like "Sans would never do this" "Sans could've handled that" "Sans is out of character in this" So, let me tell you, if you tried to kill one of your friends because of a prank your other friends pulled on you, would you keep your cool?
UPDATES:
1. I made it so he didn't smash an entire wall, but only a pillar.
2. I made it more descriptive and gave Undyne the appropriate reaction. Her's was too soft in the original Ch. 1.
Sans buried his face in his pillow. That was the worst prank anybody could ever pull on another person... monster... skeleton. Sans was embarrassed, but he would never admit that. He heard whispers outside, but he couldn't tell who the voices were from. He could tell which one was Papyrus's, however.
"Do you think we went too far?"
"Yeah, he's definitely going to kill us..."
"I DON'T THINK HE'S OKAY," That was Papyrus.
"We should knock,"
"If we walk in, it's not going to change the fact he's going to kill us,"
"Undyne, I told you..."
"Is he in there?"
Sans took a deep breath and laid down. He was tired, anyway, despite the adrenalin. He used a lot of energy back there, especially for someone who's regular schedule was talk, eat, work, eat, eat, sleep. Trying not to brutally murder his friend was out of the ordinary. At least now they knew how to make him angry.
Probably not, actually. Well, maybe. He didn't pull off the surprise well, in a panicked daze he turned the attack to the side and blasted a, thankfully decorative pillar.
He turned in bed and dug his face into the pillow further. He faintly heard Papyrus outside his bedroom door whispering that they should let him go in first. Whatever. Didn't matter anyhow. Sans barely cared enough in his semi-panicked mind to note this was the quietest Papyrus had ever been.
Papyrus knocked. Then he knocked again. Sans wondered if Papyrus would get the hint. Probably not. Probably best not to treat his brother lowly. But after all he's done for Papyrus...
Papyrus stopped knocking and whispers broke out. He'd never seen Sans so angry before, or, well, not so much angry as scared or surprised. An abrupt change in mood, he was usually putting a happy face on.
Well, what did he suppose would happen? Papyrus only picked up a few whispers, lost in his own thoughts momentarily.
"Is he even there?"
"We could leave him alone..."
Sans listened as Papyrus knocked once again, and even considered letting him in. However, if he didn't respond, he could play it off like he sleeping and avoid explaining himself. After his friends got over the shock, they'd most likely be angry.
Undyne smashed the door open, breaking it into pieces and making a deafening noise. Not a nice noise to go to sleep to. Sans didn't even dare to change positions.
Undyne stood over Sans, holding a spear. She impaled it into the ground and leaned her head on it. Papyrus took a cautious step inside and the rest followed.
Against his own thoughts, Sans sat up. He looked at them, partially dazed with the combination of surprise and anger. He didn't show that on his face, though.
"Sorry, Sans..." Undyne mumbled, avoiding eye contact with a subtle angry expression on her face, like this was his fault. She didn't like apologizing, 'Don't be such a big baby' was what she would've said to him if it didn't damage her reputation with her friends more than the incident already had.
Sans didn't say a word and just laid back down and pretended to sleep. He heard a few panicked words from Alphys. He wasn't so bad an actor after all.
"He's fainted. H-He used too much energy back there," Alphys nervously twiddled her thumbs. Does she have thumbs?
Sans resisted the urge to smirk. He could show them all how much energy he really had if he hadn't been taken by surprise, and if he wanted to. Which he didn't, despite how evil they acted.
"We'll just have to come back later then goodbye," Undyne stuck a hand in the air and walked out of the room. The spear she had placed glowed blue then disappeared.
"...I don't think Sans likes it when we watched him sleep," Frisk led Papyrus and Alphys out of the room, face still flushed from the experience. When she overheard Undyne mention a prank, she didn't think too much of it. Undyne was always angry at Sans' traps. The most recent one had been a whoopee cushion prank, which Undyne stood up, put it on the end of her spear, then slapped Sans upside the head with it.
Frisk had really thought that was the end of that.
-=-=-Sorry-=-=-
Papyrus was worried. Sans had been asleep for 3 hours now, and had even asked Alphys about it. Alphys said that he was probably most likely maybe fine, and after a couple more buggings, estimated he would be asleep until tomorrow morning. Because that's how people sleep. They go to sleep, then they wake up the next day.
Also, Sans had slept for 2 days in a row once. He missed Thursday last week. Why freak out after 3 hours? They were all a bit antsy, especially Undyne, who wanted the situation over with, but...
Well, actually, Undyne was the least worried about the situation. She still had a bit of soul in her, which meant she was a bit worried, but Alphys, Papyrus and Frisk were all worried more.
Undyne was just annoyed the prank she planned didn't turn out better. Sleeping wasn't a very good "gotcha". Undyne felt a bit bad for tricking the human, however. We'll take a walk to Asgore's old home, for memories. Oh, look, something behind the rock, we'll check it out, you stand there. Sans walked in.
Undyne went with Alphys back to Hotland. Papyrus worked nervously on his puzzles all night. Frisk took a while to get to sleep, but she finally did, after tossing and turning for a while.
The next morning, Papyrus called Undyne and Alphys, whom were still asleep, and forced them to come over. Undyne was reluctant, but Alphys was almost excited.
Frisk woke up next, and walked downstairs nibbling on a donut she had bought the day before. She still didn't have much of an appetite, much less for the spaghetti. Although, he was improving.
By the time Alphys and Undyne arrived, Sans was still in his bed. He wasn't sleeping, although, after pretending, about an hour later he did fall asleep. He had woken up, startled by Papyrus yelling on the phone. He wasn't easy to wake up, but... you know.
He sat there for a couple minutes that felt like hours. Well, he had to come down and eat, didn't he?
Sans got up and opened the door, walking downstairs. Papyrus reacted suddenly and ran to his brother, giving him a tight hug.
Grateful but at the same time annoyed by his brother (Which he didn't get annoyed at often), Sans decided to play it cool, at least until the kid got out of the room.
"what's with the hug?" Sans smiled.
"YOU WERE ASLEEP FOR A YEAR!" Papyrus was happy and exaggerated greatly. Sans had been asleep for 10 hours, if he went to sleep at 9:00PM and got up at 7:00AM. Well, he got down the stairs at 8:00am, but still, that's only 11. He usually slept for 12.
Sans reacted, sticking to playing dumb.
"what? why? oh, and kid, can you go and fetch me one of those donuts? i'm hungry."
Frisk nodded and scurried upstairs, closing her door out of instinct. It also seemed pretty clear that Sans wanted to tell them something, and didn't want Frisk around for it. Sans, despite popular belief, actually preferred mustard and occasionally ketchup for breakfast. She didn't want to hear it.
Back downstairs, Alphys, Papyrus and Undyne all bought it and thought he didn't remember it. Undyne whispered a quick word to Alphys. Sans watched, a slight smile plastered on his face as to not give anything away. Yet.
"Should we tell him now and anger him again or let him figure it out by himself? It wouldn't matter to me, YOU'LL die later. I'm leaving the moment he shows any sign of remembering." Undyne cared, really. She just never really watched what she said, and Alphys understood it as a joke... sort of?
"WE PRANKE-" Papyrus said but was cut off, spoiling any plan Undyne had to escape before Alphys responded.
"I know. You really thought I didn't remember it? Not a lot of things get on my nerves, but you won the award. Congratulations. Tell the kid I grabbed a mustard bottle instead." Sans grabbed a new bottle of Mustard and walked over to the staircase, leaving everyone in shock.
He turned around, and looked back at them.
"Two more things, I'm more disappointed in you, Papyrus. I'd expect this from Undyne, but you? You too Alphys. I don't really care about it now, but I would've if you'd gone the extra mile," Sans smirked.
He went halfway up the stairs before pausing again.
"And one final thing, the kid didn't know about it, now did she?" Sans smiled, eyes dark.
"N-n-no..." Alphys responded nervously, backing up into Undyne, then, turning red, moved next to Undyne instead.
Sans sighed, his pupils returning. "I'll be in my room,"
He finished the route to his room, and giving them a final stare, closed the door.
He didn't know what to think. Once again, he'd pulled off the "I don't care" attitude, but how long would that last? He really was hurt by it. He pulled pranks on them, but he never did anything like that. Oh, well. He had to find a way to get over it, soon, before Undyne gets any idea that he was affected and does another, similar prank.
Frisk finally walked out of her room and down the stairs. She looked at the shocked faces of Papyrus and Alphys. Even Undyne was a little shocked. They whispered a few words to each other before looking back at Frisk.
"I heard everything." Frisk was pale.
"We broke Sans."
What do you guys think the prank was? I don't care if this was posted three years ago, you're always free to guess. A hint:
It included him almost killing innocent Frisk, which was more than enough to make him lose control and shatter a pillar. It must've been really bad.
You could probably go to the next few chapters and find out, but still. It's always fun to interact with the story.