dream1990 gave me this idea for a fan fiction. Enjoy. (Or don't and waste your time.)
Sans' pov.
I have visited Toriel today. It was like the other times. We shared jokes, told each other stories, Papyrus finding me and saying how lazy I am. Except Toriel seemed sad. When I asked her what was wrong she told me about failing to protect the fallen children as well as her own. That is how I found out about Chara. The first fallen human. And about death of the Prince. I wanted to help her but I couldn't do anything. Until today. I tried to repair the machine that was behind mine and Papyrus' house. If this worked then I could go into the past and change everything.
"BROTHER? WHERE ARE YOU, YOU LAZY SACK OF BONES GET BACK HERE."
I teleported behind Papyrus.
"hey bro. what has gotten you so rattled up?"
"WHY ARE YOU NOT AT YOUR POST? WHAT IF A HUMAN COMES BY? WE NEED TO BE READY."
"bro. there is something I need to tell you. this is not the first time you said that to me."
"BROTHER. WHAT ARE YOU TALKING ABOUT?"
"this human that is going to fall soon can control time. they have determination. they can die and come back to life. it doesn't matter. that is what i am trying to fix. this machine behind our house can go back in time. i have tried for so long to fix it. just give me some time. i will get it done."
"BROTHER. WHY HAVE YOU SAID NOTHING THIS WHOLE TIME? I, THE GREATE PAPYRUS SHALL HELP YOU SOLVE THIS PUZZLE."
"nah bro it is alright. i'm almost done anyways." Papyrus is always eager to help but he knows nothing about science or technology.
"WELL THEN. I SHALL HELP YOU BY MAKING THE BEST SPAGHETTI THIS WORLD HAS EVER KNOWN." With that he went to the kitchen.
After some more tinkering, and some terrible spaghetti, I have finally finished.
"BROTHER? ARE YOU ALRIGHT? DO YOU NEED MORE SPAGHETTI?"
"nah bro. i'm good."
"IF YOU SAY SO."
I was finally ready. I grabbed some burgers and fries, some extra ketchup, and was finally ready to go back to the past. I said my goodbyes and pulled the lever.
"BROTHER WAIT FOR MEeeee..." Then everything faded to black.
I woke up and looked around me. The machine was still there. My box full of food and ketchup was still in my hand. But the room was empty. I teleported to the ruins and knocked at the door.
"Who is there?" It was Toriel's voice.
"orange." It was the best joke I could come up with at the moment.
"Um... orange who?" She asked. She was definitely crying earlier.
"orange you glad to meet a new pal?" She giggled a bit.
"How can you make jokes when my son died yesterday? And my adopted child Chara was unwell for a week and died today as well." Dang in. Good job Sans. Well at least now I know how far to go in to the past.
"i'm sorry about that. i just travelled back in time and I didn't know when they died. i am here to fix that and prevent their death." She probably won't believe me but it won't matter.
"Are you serious? Who are you? What do you want?"
"it doesn't matter right now. but i am here to help. just tell me the place where your children were before they died and i will try to help them."
"I don't think you can do anything to help him. But if you are saying the truth then they were in the kings castle. If you can really go back in to the past then tell my past self that I hid the knives under the bookcase. That is something only I know."
"thank you toriel."
"How do you know my name? I don't recognize your voice."
"you will know in the future." I teleported away and inserted the time in my machine. Then I pulled the lever.
I was back at the basement. I went outside and looked around. Nothing unusual. I teleported to the kings castle and knocked at the door. My non existing heart was beating fast. I was actually really nervous. But at least I don't have a past version of myself to walk into. What could go wrong? The only monster that could possibly recognize me was Gaster. But Gaster was always in his lab.
The door opened and I saw Toriel.
"Hello. Pardon me but I do not recognize you. Who are you?"
"my name is sans. i know this might sound crazy but i am from the future and in the future both of your kids die. also your future self told me to tell you that you hid the knives under the bookcase. you said that it was something only you knew."
She stood there with a loss for words. Then she finally managed to say. "How did they die?"
"i don't know. but chara was unwell for a week. then they died and your son died on the same day. that is all you told me."
"But you could be lying"
I stoped time, teleported to the machine, got my ketchup bottle, then teleported back and resumed the time. I showed her the date the ketchup bottle was made.
"this bottle of ketchup was made yesterday by my time. but as you can see that is not the case for you." She took the bottle and looked at it carefully. Then turned it over and looked at it again.
"How?"
"i fixed a machine that was made by w.d. gaster. but he was split into two monsters. me and my brother. he is still alive in your time but soon he will fall into the core."
"Then we must warn him."
"no. if he doesn't fall into the core then i will never exist. if i never exist then i won't go into the past which means that no one will warn him. but that means that he will fall into the core. all that will cause a paradox which will destroy this timeline."
"What do you need?"
"i need to talk to your children. perhaps i can prevent their deaths."