It's been an eerily silent genocide run in the underground. Very few monsters have appeared, forcing you to perform your run with very low LV, and additionally low HP. Hours pass, and you reach the stage area of the Underground's Core.

Where you usually encounter Napstaton, you find someone else entirely...

A familiar, short, skeletal figure stands across from you on the stage, with his back to you. It's Sans... Papyrus' hoodie is tied around Sans' waist by the sleeves. In each hand, he brandishes a purple sword, and his clothes are littered with dust. He doesn't turn around to look at you just yet, but he knows you're there.

After a moment of silence, Sans speaks up.

"If there's anyone you care about... Make peace with yourself now..."

A series of purple bones rise from the ground and block the exit behind you.

"...Because you're never seeing them again."

Sans turns around to face you, a wicked grin plastered to his face, and a large crack on the top of his skull. His eye sockets are dark, and empty.

Any kindness, mercy, and belief Sans had for you in any previous timelines was gone without a trace.