This is just based off some Undertale stuff I saw on YouTube-
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Briki was walking through Waterfall. She was covered in Monster Dust from all the Monsters she killed.
"Well, this is definitely something. Never really experienced killing. Guess it ain't so bad." Briki shurged as she walked towards the area of the Undyne fight.
She suddenly hears footsteps behind her and turns to see who's following her. It was just Monster Kid. Easy kill and exp, right?
Monster Kid ran up to her. "*Yo! Undyne told me to stay away from you. She said you...You hurt a lot of people... But, yo, that's not true, right!?" Briki turned away, not wanting to hurt the poor kid. She hoped turning away would scare the kid so he'll run away, but it didn't work...
"*Yo... Why won't you answer me? A...A...And what's with that weird expression...?" He asked when she didn't answer and had a weird expression of internally fighting herself. Monster Kid seemed to be doing the same as well.
"*Oh... Oh man... Yo... Y-You'd b-better st-stop r-right where you are... Cause if you w-wanna hurt anyone else... you're... You're gonna have to get through me, first. A...an...and..."
The battle had started. Briki looked at the kid. He looked like he was gonna fight, but was scared to. Briki's inner demons kept telling her over and over to kill the poor kid, but, after a while of internal fighting, she decided to spare him, thus ending the battle.
"*S-see, I knew y-you weren't so bad! I've gotta go tell Undyne she was wrong about you!" Monster Kid runs off.
Briki smiles to herself and walks away to get ready to probably fight Undyne. Briki walks up to the Undyne area and sees her waiting there. Undyne starts monologing...
"*Seven. Seven human souls, and King ASGORE will become a god. Six. That's how many we have collected thus far. Understand? Through your seventh and final soul, this world will be transformed. First, however, as is customary for those who make it this far... I shall tell you the tragic tale of our people. It all started, long ago..."
"*... Forget it. Look. Papyrus didn't come to his meeting today. Say what you want about him. He's weird, he's naive, he's self-absorbed... But Papyrus has NEVER missed a meeting. And no matter what time you call him on the phone... Night, day, afternoon, morning... He ALWAYS answers within the first two rings. ... But now he's gone. And his brother isn't around, either. ... What did you do to him? What did you DO TO HIM? Papyrus, who I have trained every day... Even though I KNOW he's too goofy to ever hurt anyone... ... Go ahead. Prepare however you want. But when you step forward... I will KILL you."
Briki walks up to the save point and saves, then walks up to fight Undyne.
"*En guarde!" Undyne shouts.
The fight begins and Undyne makes sure Briki can't escape by making her Green.
"*Now that you're GREEN, you CAN'T ESCAPE, unless yoh learn to face danger head on!"
After awhile of fighting and blocking spears, Briki finally gets the last hit on Undyne.
"Finally!! You were way harder than Papyrus. He would be so happy! And you probably should've trained him better." Briki smirked.
"*A-at least I didn't go around killing EVERYONE! Killing everyone I love... A-at least Alphys will have all of the Underground evacuated..."
"Oh, I will find lots of monsters around to kill. And hey, once you finally die, you'll be with them!" Briki laughs. "Speaking of dying, how are you still alive? All your HP is gone!"
"*Did you really think it'll be that easy to kill me? I WON'T DIE! And I'm going to kill you! I'm DETERMINED!!" the battle starts again and Briki sighs.
After a few hits, Undyne starts to fade away. Briki smiles, happy Undyne is finally dying.
Undyne's body comes back together. Briki sighs. She wants this battle over already.
Undyne starts speaking. "*I WON'T DIE!" she melts a little. "*I WON'T DIE!" She melts more. Briki is startled by this. "*I WON'T-" Undyne finally turns to dust.
"She... she actually melted... Wha..." Briki stares at Undyne's dust. She didn't expect Undyne to melt then turn to dust. Monsters just turn to dust, they don't melt.
"Ah, surprised she melted? Hahaha, You're so funny Briki. It was expected!" Ikirb said as she split from Briki. She was the one that made Briki do the Genocide run by taking over her soul and showing her false images of the monsters trying to put the people that fell in the Underground in pods to break the barrier that trapped them there.
"What do you mean, 'It was expected!?'" And why did you make me kill her?! Why are you making me do this?!?!" Briki was crying on the floor. Now that she was separated from her dark self, she saw all the pain she caused from all the dust on her.
"Briki, it's simple. Monsters with Determination injected in them melt when they die. I'm sure that's why Undyne melted." Ikirb was staring at Briki. She didn't care that Briki didn't believe her now that she was separated from her. She liked watching Briki cry.
"H-how do you know that? How do I know it's not another lie!"
"Oh, this isn't a lie. Monesters aren't able to be determined, so it has to be injected in them. But their bodies can't handle it, as you've just seen."
"I-I don't believe you!" Briki screamed.
"Oh well." Ikirb sighed. "Gimme your knife!"
"Huh? Why?" Briki asked, confused.
"I'm gonna kill you, duh!" Ikirb laughed.
"B-But you can't!" Briki held the knfe close to her so Ikirb wouldn't grab it. "If you kill me, you'll die as well! And June isn't here for you to take over as well!"
Ikirb laughs once more. "You've been killing and gaining levels. That's all I need to gain control over you. And now that you're free from and you won't believe me, I ain't gonna let you reset and start over!"
"N-no! Don't k-kill me! Pl-please!!" Briki begged.
"Too late! So long good side, you were a pain to deal with anyway. I'm gonna be so happy to not deal with you anymore!" Ikirb quicly grabs the knife before Briki can react and kills her. After making sure Briki is dead and that she has the power to reset now, Ikirb leaves Waterfall and goes to kill everyone else.