A/N: Hello, there. Welcome to my first Undertale fic. This is a oneshot that came to me in a fit of...something. Detemmination? Sure, let's call it that. Cute, fluffy, friendly. I'm assuming that this will take place after a true pacifist run, but still in the Underground? I focused on the moment and not on the actual world creation here, sorry. Hope you enjoy?

THE BAD THING

"Uncle Sans," the child muttered, frightened.

Sans cracked an eyelid. Nothing like a frightened kid to wake him up in the middle of the night. He looked over at them and grunted before holding out his arm.

"What's up, Kiddo?" he mumbled, his face half hidden by the mattress.

Frisk walked with wobbly, tired steps to the edge of Sans' bed. "There's a bad thing in my closet."

Sans smirked. It hadn't taken long for the kid to stop saying there was a monster in their closet—there were monsters everywhere in their world now that they were staying with him and his brother in the Underground. Instead, they used the term 'bad thing.'

"What kind of bad thing?" he asked, taking their hand.

"I don't know, but I can hear it breathing."

That was unexpected. Sans was suddenly wide awake. He sat up on the edge of the bed and pulled them up to sit next to him. He hugged them gently.

"I'm sure it's nothing, sweetheart. Stay here and Uncle Sans will check it out, okay?"

This was the usual procedure. Frisk would have a nightmare about Chara or Flowey, Sans would check it out while they waited, safe and secure on his bed, then he'd tuck them back in. In the back of his skull, Sans couldn't help but feel like this wasn't about to be a normal night.

Frisk shook their head and grabbed his shirt. "No, Uncle Sans! The bad thing is really there tonight, I swear. I heard it. Please don't go in there." They looked up at him with pleading eyes.

He lightly kissed their forehead. "Frisk, if there's something in your closet, then I need to make it go away."

Their lips trembled. "What if it hurts you?"

He smiled reassuringly. "What? I'm Uncle Sans; nothing can hurt me. 'Sides, I'll give 'em a bad time if they try."

Sans hugged them gently. He had hoped he'd quell their fear, but it seemed they were pretty damn serious about it this time. They cried into his shirt as they clung to him tighter. He took a deep breath and patted their back.

"What if you go in there with me?"

Frisk looked up at him with wide eyes. "No!"

He smiled. "You can take your stick." He reached over to get the stick that was leaning against the wall by the head of his bed. "You can help me fight it off."

The idea of helping Sans fight off an enemy made them feel a little less scared.

"What if it hurts me?"

Sans' smile faded. "Then it will have a really bad time."

Frisk took the stick and nodded. "Okay. I'll help you. Can I go in first?" The more they thought about it, the more they liked the idea of protecting their Uncle Sans from a bad thing. It filled them with determination.

Sand nodded. "Sure can, Kiddo. Let's go check it out."

Sans wasn't so sure about having Frisk go first, but he knew he could stop whatever from hurting them if it tried. He let himself hope it was just Papyrus, but he already knew it wasn't. He wasn't the skeleton known for falling asleep in weird places like closets; that was Sans' responsibility.

He pushed the kid's door open and stepped slowly to avoid running them over as they crept into the room. He flicked the light on and smirked at them. They held their stick out like it was a sword, ready to swing at something if it moved. He put his hand on their shoulder and rubbed gently with his thumb to help keep them calm.

As much as he wanted to just get this over with, they were moving slowly. He kept their pace as he listened for any strange noises.

They finally reached the closet doors. Frisk held their sword out further and watched in horror as Sans pulled the door open.

They screamed as they were attacked by…something.

Sans laughed out loud as Frisk was suddenly covered by a pile of clothes. He watched as they struggled to get out from under the mountain.

"That wasn't funny!" they yelled.

Sans tried to shush them, but he couldn't stop laughing. "That was hilarious! Did you see that?"

They glared at him. "I saw an avalanche."

It took Sans a moment to calm down before he could breathe enough to speak to them. He leaned down to get to their height and smirked at them. He thought the blush on their cheeks was adorable; they knew they were busted.

"The only bad thing I see in here is that you didn't put your laundry away when you cleaned your room. You put everything in here!" He laughed again as he gestured to the mess now sprawling out of the closet.

Frisk pouted. "It was too much work."

If Sans didn't know any better, he'd say they really were his kid. "I know, I know. But even your Uncle Sans has to do work sometimes. Let's go back to bed, and you can clean this up tomorrow."

Frisk nodded and yawned. "Okay."

"What about this breathing you heard? Nothing could survive in here for more than two minutes." Sans poked his head in the closet; there really was no way something could fit in there.

Frisk looked at their feet. "Now that I think about it, I'm pretty sure it was you I heard. You were snoring."

Sans laughed. "That would make sense."

He tucked them in, read them a short story, and kissed their forehead. As he walked out, he made sure to leave their door open a crack.

He laughed quietly to himself as he made his way back to his room.

"SANS?" Papyrus yelled as quietly as he could. "WHAT WAS THAT RACKET?"

Sans peeked out of his room to see Papyrus standing in the hall with his hands on his hip bones.

"That was Frisk. They had a bad dream."

Papyrus looked worried. "OH, NO! DO I NEED TO COMFORT THEM?"

Sans shook his head. "I got it taken care of. Go back to bed."

Papyrus nodded and yawned before turning back to his room.

A/N: Thanks for reading! If you liked it, punch that like button in the face, like a-

Dear God. I apologize. It's late, I'm tired, and I may have a small crush on a youtuber several years younger than myself. Shut up; you do, too. Most likely.

Reviews are lovely.