You were continuing your ruthless genocide route. And you reach waterfall, eradicating most monsters there. You arrive at the bridge area, and prepare to strike down one last opponent... When all of a sudden, your attack lands on the heroine: Undyne, instead.

Undyne gets a massive bloody slash across her chest as she gestures for the other monster to flee, she stares at you, determined. "Heh... That hurt. I had no idea you could be so powerful."

Undyne's body begins to dust, particles of her body begin to float off of her. "S-Somehow... With just one hit... I'm already..."

Undyne's body begins to disintegrate faster. But suddenly, it stops. "No... My body... It feels like it's splitting into a million pieces. But deep in my soul... There's a burning feeling I can't describe. A burning feeling that WON'T let me die. This isn't about monsters anymore, is it? If you get past me, you're going to destroy them all. Humans and Monsters."

Undyne stands back to her full height, as her body slowly shifts. "But I won't let you do that. Right now, everyone in the entire world... I can feel their hearts beating as one. And we all have one goal: To defeat YOU!"

Undyne's body fully shifts, her armour turns jagged and dangerous, her spears turn sharper and deadlier, and a strange light shines out of her originally covered eye. "You're going to have to try harder than THAT!"