Where in Undertale does it ever say that Sans has “karmic retribution”

I've noticed quite a lot in comment regions on youtube and such mentioning that Sans has this thing called karmic retribution which means people who have committed sins take more damage from him. Now, I've never played undertale (far too difficult for me) but I have watched 2 entire playthroughs of it all and try to keep tabs on it heavily. I'm confused where this comes from in game. I've never seen Sans or Flowey ever mention this (I would only assume to come from them as nobody else would have first… petal/bone experience).

Now I know what they are referring to (his poison attack that drains health), but I am confused where they get the idea that it's karma and not just a poison attack.

Best Answer

It doesn't necessarily say "karmic retribution" in the game. See the wiki page:

All of Sans's attacks use KARMA (KR). This effect removes invincibility frames, and, in turn, delays damage input. HP cannot be reduced below 1 while KARMA is in effect and is depleted in the place of HP when taking damage; if the protagonist suffers otherwise fatal damage while KARMA is in effect, it effectively becomes a decaying second health bar. Purple is the mode and color of the purple SOUL and is associated with Trap Mode and perseverance, but it is unknown if this is associated with KARMA.

Karmic Retribution is a common phrase that refers to when someone gets what is coming to them. Since you only fight Sans if you've been killing NPCs, his attacks use KARMA, and he is by far the hardest fight in the entire game, it makes sense that people would refer to it as such, even if that's not technically what it is called in-game.