zarla-s:

I was thinking about Sans’s poison effect (KR) today, and one theory is that KR is directly fueled and/or connected to your evil deeds, so the KR is turned on by all your murdering. If that’s the case though, that means Sans’s threat during your date in a neutral/pacifist run is a little toothless, because if you haven’t been murdering anyone, he’s only going to do one damage. Though he may still have the INV-skip, haha. While it’d fit thematically for him to only be dangerous (or for you to make him dangerous) on a murder run, I think that the KARMA mentioned in his battle is in a more figurative or metaphorical sense than literal and KR/INV-skip is just something he always has.

Re Sans battle, if I remember correctly, the reason you don’t get invincibility frames is because his attacks don’t deal any regular damage, they just convert HP to KR. Like if you touch an attack for only a few frames, you accumulate KR, but you don’t lose overall health if you don’t account for KR draining. The exception is when you’re out of regular HP and your KR drains at 30 per second.

zarla-s:

[SPOILERS] Now that’s an interesting point! That would definitely explain how KR and INV-skip are connected and why Sans has the INV-skip in the first place – he skips your INV frames because he’s not actually hitting you, he’s just poisoning you! All the damage you take in his battle is solely due to you accumulating KR! Which would again fit the theme of your sins coming back to haunt you and the messages you get in the battle about death from KARMA – it’s your own evil deeds that kill you, and nothing else!

It’d fit his role as judge too – the power of KR becoming in itself an implicit judgment of you and your actions, he himself only a vessel for that judgment’s power. AND relying solely on KR for damage would match Sans’s pitiful stats and the text saying he’s the easiest enemy, AND it’d explain why he’s so incredibly reluctant to fight you until you pretty much force him to. It alters the subtext of his early warning in the run – “if you keep going the way you are, you’re going to have a bad time” meaning if you keep doing these terrible things, you will fuel the KR that will eventually kill you. Even his gloating when he keeps track of how many times he’s killed you then takes on a different light – it’s not him that’s killing you, it’s you in a sense, no wonder he’s so smug about it.

The idea that KARMA is something that YOU created and is something YOU brought on yourself through your own choices and determination fits with the themes of responsibility and consequences that run through the whole murder run. And it also fits the definition of karma to begin with. This all hangs together surprisingly well! All this points to a neutral/pacifist run Sans being… pretty much a non-threat, just like I joked, haha. Maybe Sans not doing anything when Flowey grabs everyone at the end of the pacifist run makes sense after all.

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There is the very very end of his battle though, when he starts throwing you into the walls in desperation. In the no-hit run, you can see that he can do 36 damage in that last frantic burst of energy, damage that isn’t KR damage. Previously in the battle, he throws you into walls the same way during certain attacks and it does no damage, so obviously this is different somehow (it’s also one of the few times during the battle you see his eye glow, the other being the very first time he throws you down in his opening barrage… though he does no damage there either).
But interestingly, he can’t kill you with that attack, he can only bring you down to 1 HP. So if you got to that point with like 2 HP, he’d hit you for one damage, then all the rest of the strikes would do no damage at all (as seen here). So what exactly does that mean? Could he do that attack in a neutral/pacifist run and be threatening that way? Would he? I get the impression that that’s really his last resort and he’s burning everything he has to do it – he wouldn’t do it if he had any other options. But even if he did do it in a neutral/pacifist run… he couldn’t kill you with it.

This fits with the description saying he only does one damage… each strike against the wall only does one damage to you, and it’s the only non-KR damage he does. And even then, it literally can’t kill you. :< I guess in that sense, he and Papyrus are the same.