Frisk shuffled her feet along the carpet, moving as quickly as she could from her bedroom to the living, where Sans was binge watching some show on television. She climbed up onto the couch and shoved her tiny body against his side, whereupon Sans laid his arm around her as if it was an everyday occurrence. They sat in silence for a few moments, watching the characters on the screen act out their meaningless roles.
"I had a nightmare," Frisk said suddenly, quietly.
"I know," was all he said in reply.
Frisk was not old enough to possibly comprehend what Sans had acknowledged, so she sat on the couch beside him until she fell asleep once more.
Inwardly, Sans turned to his own thoughts, where he was remembering his own experiences with the nightmares he developed as he grew up. When he first dreamed of a different time line.
*flashback*
"Sans! Sans! Wake up!" a tiny voice squeaked, rattling Sans' bones as he woke him.
Sans jerked awake, slightly frightened by the voice, but also from the sudden reverse to reality. He was confused as he looked at his younger brother, who beheld a sad expression.
"What is it Papyrus?" Sans asked drearily.
"You were crying," Papyrus said as he shuffled his feet and looked at the ground, pulling at the fabric of the cape attached to his pajamas for that particular night.
"Don't be silly," Sans replied. "Go lay your bones to rest."
"If you say so brother," Papyrus said, leaving his brother alone to return to his own bed.
Troubled, Sans laid back down when the memories of his nightmares began to flood back into his mind. They had recently begun, and he still had yet to figure out what it all meant. All he could see was this human in a striped pink and blue shirt. Some nights it would be a good thing, like the human laughing with an older Papyrus, who still wore costumes and still preserved a naive personality. Other days, like this one, he would wake up to Papyrus shaking him awake. This wasn't his first nightmare.
The first one was a year ago, when Papyrus had left to stay with Undyne for the night. They were having some fishy sleepover or something. Sans was left alone in his room, in the empty shell of a house. He could only remember fuzzy details now. His first dream was of the human and his brother. Sans saw the tiny human approach Papyrus, who was dancing around and claiming that he was going to kill her, which he knew would never happen since Papyrus couldn't even squash a fly. The tiny human stood at Papyrus' knees as he danced around her, taunting the human and waiting for retaliation. That was when his dream went black. The next thing he saw was dust littering the ground at the feet of the human. The human only held a knife, with a gruesome smile plastered on her face. But that wasn't the only dream.
There were many more dreams after that. Sans watched many people die at the hands of the innocent looking human. He watched people he didn't know suffer. He watched as Undyne fought with all her might, but with one strike did the human knock her down. Sans watched numerous times as the human killed Dr. Alphys, after she had tried to guide her through part of the Underground. He watched with great despair as she annihilated monster after monster. The worst part of it all was his brother. It hurt to see him die at the hands of a human. He could only dream of the revenge he was to take on this human.
That was when his dreams changed. He then began to see the same striped human be loving and kind to these friends of his. There were dreams where she hugged all of them and attempted to make pasta with Undyne, after his idiot brother had jumped out of a window. There were dreams where she lived with Papyrus and himself, and laughed at his amazing puns and snuggled with Papyrus as they fell asleep. That was beautiful, that was the human he wanted.
Eventually, Sans grew up and as he grew older, his knowledge of these dreams expanded. He could feel the closeness of how the human was coming to reality. It was as if he was living and breathing in different timelines besides the one he was living in right now. It affected him, but he didn't let it show. Sans had to protect his brother, and all of his friends. When the time came, he swore to himself he would make the right judgement. He would protect his friends, his family.
Now that he was here, sitting with a tiny Frisk on his lap, he knew this was not over. He felt, that even though the human did no damage here, she was doing something elsewhere. Sans was afraid, even though he still had all his friends and his brother, that there may be another human, another time, another place where he could lose his brother. But for now, he sat, watching his stupid comedy and held onto Frisk as she slept peacefully. For now, he would live in a slight bit of happiness, knowing that everything was alright, that life would be just fine.