You left the Ruins after killing Toriel and the resident, finding yourself in Snowdin, as usual, but Sans wasn’t there at Papyrus’s gate. Weird, but okay. Going to Waterfall after killing Papyrus, you also didn’t see Sans at his stand either, but of course, it’s just because you’re doing a genocide route. Moving to Judgement Hall after exterminating everything, you see the entire corridor is surrounded by vines. But, who could've done this? Only Flowey is responsible for making vines… OH!

BATTLE, START!

You’ve been ambushed and forced into a battle with… Sans? What the hell happened to him? And Flowey. He’s taken Sans’s right eye!

Flowey: “Heeheehee… hello, “partner!” Look at what I’ve got in my hand! Indeed, a Sans! Oh how much of an IDIOT you are. You should’ve killed me the moment you could. HAHAHAHHAHAHAHA. But now, that’s not important. Remember what I said? D I E .”