Character: undyne the undying

Undyne's transformation which is only seen during Undertale's Genocide Route. It is widely considered one of the hardest encounters in the game.

In Waterfall, the protagonist attacks the Monster Kid and Undyne takes the damage instead. Her HP hits 0, but she persists with her incredible Determination, and transforms into Undyne the Undying. Her pupils turn white, and her sclera turns black. She no longer has an eyepatch, and her newly revealed empty eye socket emits a flashing light in the shape of a spear. Her hair spikes at the ends like a trident and her fin rays have become more rigid and pointy. She also appears to have donned a new set of darker colored armor with souls/hearts on her chestplate and new white gloves, which have spikes protruding from their backside, accompanied by boots with pointed toes.

This tag implicates undyne (learn more).