This is just something random that popped into my mind. Frisk is just out of toddler-hood, 4 or 5 at most. No Chara-infestation, and Flowey doesn't play a part. Just Sans being... well, Sans. Frisk, at least in this version of my personal headcannon, has a severe sensitivity to light (squinty-looking sprite in-game anyone?) and a selective mute due to reasons. Also her nightmare isn't anything other than that, just a baby's mind running rampant.
Crossposted to tumbler and AO3.
He jerked awake, nearly throwing himself off the bed. How a monster that didn't need to breath would wake up gasping was just one of those mysteries of the magic intrinsic to their world that he would someday get around to questioning, but not at oh-dark-thirty.
His screams echoed in his ears, and he snapped his jaw shut with an audible click. Had Papy been around, he'd have been scolded as was soothed, but his brother had decided to spend the night with Mettaton. They kept echoing, and his groggy mind took a moment to process that these wordless wails belonged to someone else.
The back corner of his mind questioned why he slept with blankets since he didn't really have temperature sensations as he struggled to free himself. A quick teleport saved him from an inelegant dumping onto the floor, and he pushed himself up and hit the hallway at a run.
The hall light was on already. it always was. Dim shadows fell lazily across the doorways lining the hallway as he skidded to a stop just outside her ajar door. The kid's eyes were so damned sensitive that going from a dark room to a simple bedside lamp was like socking her physically in the gut.
He tapped the door, and, not expecting a response, let himself in. The bells tied to the interior doorknob tinkled quietly, their sound drowned out by her sobs.
Frisk was sitting in the corner of her bad against the wall, arms wrapped around her knees as she cried, tears soaking into her pajamas. Eyes clenched closed, she was rocking back and forth, lips moving soundlessly. He was pretty sure she wasn't aware of him yet, so he purposely made his bones rattle as he settled onto the edge of her bed.
She hiccuped, one eye cracking open so a tiny sliver of green peered out, then shut again, more tears staining her face as they slid down to soak into her pajamas. Sans shifted closer, resting a hand on her knee.
"Hey kiddo, you wanna talk?" he asked gently, rubbing a circle with his thumb across her kneecap.
Her screaming nightmares followed a set sequence of events. He and Papyrus would alternate telling stories until she would slowly tell them what had scared her so badly, and she would inevitably wind up on the couch with one of the skeleton brothers curled around her.
Her lip quivered and she broke from routine, throwing herself at him in a desperate hug. Eye glowing faint blue, he gathered her up onto his lap and readjusted himself to keep her as comfortable as possible. He wasn't the largest monster by far, but she was impossibly tiny cuddled against his chest, hand fisted in his shirt.
They sat like that for hours. He occasionally murmured soft nothings to her, she occasionally cried into his shoulder. At some unholy hour she finally settled. Shifting just enough to see his face, shit bit her lower lip. It was a bad habit she had developed somewhere along the way when she was trying to force words out, and would often leave her lip a bloody mess.
He put a finger under her chin to raise it a fraction more. "Babybones, you know you don't have to tell me if you can't. Not going to rush you sweetheart." Honestly, he didn't know how much tighter his emotions could get wound.
Green eyes open now, she blinked at him, the tiny bit of light coming from the hallway glinting off them. "Was alone," she mouthed, so softly her breath barely registered. "Scared." She reburied her head into his now moist t-shirt, mostly muffling her whimper.
It took a bit to suppress his own dark chuckle. Seems like he had shared a nightmare with the kid. Good thing Papy wasn't here after all, he'd probably try to force feed them both some therapy spaghetti.
Instead, he squeezed her a little tighter, resting his chin on top of her head. "I've had that one too, kiddo. It's not fun, but you just have to remember you're not alone, OK?"
A slow head bob. Progress, at least. Toriel insisted that repetition would be the only thing to break the kid from the cycle she was stuck in, but that it wouldn't be a fast, or linear, process. Sans was willing to take any bone the kid threw him.
They sat like that a while longer, and Sans felt the siren's call of sleep pulling him. He might have actually dozed for a few moments until he caught a few more whispered words.
"Stay with m-me?"
He smiled, not that she could see it, and rubbed her back. "Of course babybones. Tibia honest, I'm pretty comfortable right now, and since I'm so lazy..."
She shook against him, fighting the urge to outright laugh at his terrible pun, and he took that as an invitation for an easy followup.
"I'm glad you always find me humerus."
She groaned at that one. Those had been some of the first ones he had taught her. His look of pure affrontedness when he realized he didn't know what a tibia or a humerus was had been, well, humorous.
He shifted so he was prone next to her, letting her choose how she wanted to be positioned. Squirming, she cuddled against him quickly, nestled back against his chest, eyes blinking heavily. Not willing to disturb her comfort, a tendril of blue magic floated over her blankets, and he cocooned them both.
She was breathing heavily just a few moments later. Pressing a soft kiss to her downy hair, Sans himself followed her down to dreams.