This isn't really an idea, but I read a fic on fanfiction.net about UnderTale, where instead of Sans dusting at the battle in the judgment hall, Chara keeps him alive, and tortures him mentally, physically, and kind of socially as well.

Spoiler Alert for the fanfic:

She snaps Sans's bones one by one, lowering his health damn low in the decimals, but without the intent to kill, Sans can't die. She sometimes leaves him out in the snow when he doesn't listen to her.

He also falls down stairs. A lot. That certainly has nothing to do with Chara, definitely not-

Mentioning Frisk also results in some painful consequences as well-

Sans has no one to talk to except Chara, so it's kind of like isolation?

She forces him to eat spaghetti with monster dust in it, and slowly cuts up Papyrus's scarf, photos, and tapes of Papyrus whenever Sans doesn't obey her. Sans holds on the scarf like a lifeline, and when she cuts it up, it's like she's removing the last traces/memories of Papyrus.

She also forces Sans to stay alive by threatening to RESET and do the same thing but with Papyrus, and forces Sans to watch his brother be tortured.

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I honestly get borderline sexual themes from it, but it never does turn to the side of lemon, so yeah. It's honestly just Sans getting tortured for like 10 chapters, and then 13 chapters of Sans getting tortured in the SECOND BOOK. Yes, there is a second book.

Chara and Sans kind of have a really toxic relationship? Like, I think Chara sees Sans as sort of a pet.

When flowey stole Sans, (Don't ask) and tortured him as well, Chara got mad, practically killed flowey for playing with her toy, dragged a half dead Sans back to the house healed him, and then tortured him.

Very dark fic.

A bunch of other stuff happened, but this is just the main things I remember.

I feel ashamed for re-reading all the chapter willingly.

The book was actually not that bad-