Welcome to an expriment I like to call 'sadism mode'! Perhaps it's a hard mode variant of the genocide route, or maybe it's just a fun game for nice children. Regardless of what it is physically this mode is designed around making you feel terrible.

Let's start with Undyne. Hero among monsters, unrequited lover of Dr. Alphys, head of ASGORE's Royal Guard. Her fight in the normal mode of the game is pretty tough, but we'll be looking at Undyne's genocide run form: Undyne the Undying. This form activates when the heroic fishlady protects monster kid from you just trying to collect free EXP and is considered to be a very tough fight. Unlike the other bosses that die in a single hit during the genocide route, Undyne the Undying actually takes a ton of hits to bring down.

At the end of the fight, we see Undyne begin to melt away as a side effect of the DETERMINATION inside her since monsters can't handle such large quantities of the stuff. She gets this way only when you deplete her energy to battle by FIGHTing it out of her, so that her body can no longer handle the DETERMINATION. That's normal for the genocide route and all, but what about something else..?

What if instead of fighting her, you laid down your weapon? You dodged attack after attack, hearing the howling wind as countless spears fired at you. If you do get in close enough to FIGHT her, it's a tiny scratch of damage. Your goal isn't to kill her, after all. What kind of human would do that?

You ACT like you know you should. You thank Undyne for training Papyrus how to fight, giving him the courage to want to join the royal guard. You thank her for putting him in your path to try and capture you, and you thank her again for being such a poor teacher to make Papyrus easy to kill. The bullets get unfair. Undyne appears to begin sweating.

You let Undyne watch your gaze turn to the hidden camera. She watches you wave at it. You tell Undyne that you're going to kill Alphys if she doesn't STOP you here. Her movements become more ferocious.

You continue to taunt her. Despite her ferocity, Undyne seems to be slowing down a little.

You urge Undyne to kill you. You yell at her to finish you off, to save humans and monsters from your killing spree. Undyne's movements begin to wind down, her 'sweat' becoming more visible as her own essence melting.

You tell her again how you plan to empty the underground, just because you want to see what happens. She never gives up.

But that's what you wanted, wasn't it? Your goal was never to kill Undyne here. You tell her that. You tell Undyne that she's the warrior who NEVER gives up. The only monster to handle such DETERMINATION in the face of adversity. She continues throwing attacks at you, but her melting form is becoming too much to handle. She's left immobilized and powerful.

That's what you wanted all along. Funny, isn't it? The very power that got you so far...DETERMINATION, will be your downfall. You watch as Undyne's body can no longer handle the massive quantities of DETERMINATION you've imbued into her by your taunts and threats, beginning to melt more severely. She can no longer move or attack.

That isn't quite the end of our story yet, though.

You look to the end of the bridge where the monster kid watches fearfully. You tell the monster kid that Undyne's body is melting away unless it can obtain more physical matter: another living monster's body. You tell monster kid that Undyne will die if this continues.

A horrified Undyne is left to watch her fate as the monster kid approaches. You watch as they shakily step to Undyne's melting body. She tries to shake her head and warn them, but her voice is too far gone.

The monster kid jumps forward, colliding with Undyne's body to begin fusing with it. Pulsating flesh can be heard as an anguished, inhuman scream rings out. Or was it two..?

Undyne's body becomes horribly disfigured as monster kid's essence dissolves into her. They melt together into a faceless, bodiless, soulless puddle. How unfortunate, it seems like it wasn't enough to reverse the process after all.

The obstruction is cleared.