As you're about to walk through the huge cave at the end of Waterfall, you hear a loud, commanding voice from above

"STOP RIGHT THERE, HUMAN!" As you look up, you see Undyne standing at the top of the cave in her usual heroic stance.

"Don't you DARE take another step forward, unless you're looking to get your ass beat! You see, human. Us monsters need YOUR soul so we can finally cross the barrier. And I'm gonna get that soul, even if I have to rip it right from your body!"