This is the fight from the Undertale AU known as Underfell. Undyne confronts the kind and caring human at the end of waterfall, and the true battle begins. Underfell is an AU where monsters that are originally innately good are innately evil, and vice versa. This means that Undyne, rather than killing to help monsterkind, is doing it for sport and to show off. This is that fight.
Credit to Pococikapusta for the Simple Undertale Battle Creator. Music is "Battle Against A True Villain" by RedInkling.
