Undertale AU Sibling Oneshots

Killer! Sans x Reader

(It totally didn't end up angsty and technically an origin story for Killertale! Reader)
Requested by the lovely corvid, fiercespiritwolf !~

It wasn't fair.
Why did that little... heathen get to decide everyone's fate?
And to rope your brother into it by driving him mad and controlling him?
No.
You wouldn't stand for it!
You avoided Sans' normal routes, taking up a new route altogether. You never took the same route twice. You last saw Sans wandering in Snowdin, and so you took the opportunity to track down the "child".
They would often return into the forest to try and find survivors, and you should in theory have no trouble finding them.
With enough luck, you found them, standing over the body of a Dusting monster. A moment later their hand was cut as you knocked their blade up into the trees with a bone.
Their red eyes locked onto you, blazing with anger.
"What're you gonna do about it, baby-boo?" You cooed mockingly, watching them rush at you with a nearby stick.
You simply teleported out of their range, firing multiple attacks at them. They dodged your attacks, trying to slash you with this stick.
Lifting your arm, you blocked an attack from the stick, watching it snap in half. You scoffed, taking the moment of vulnerability to stab through the human's chest, watching their body sit limp, suspended in the air by your attack.
You turned at the soft crunch of snow, seeing Sans standing there, looking a bit winded.
"You stopped them?" He breathed, staring at the human corpse as a bit of life returned to him.
You nodded, running over to hug him. "And if they reset, I'll stop them before they even get to you!"

Your promise was beginning to be harder to fulfill than you thought with each reset. You'd either get killed, miss their entrance to the forest, or lose them in a chase through the woods and find Sans already turned.
And dammit you were getting pissed off!!!
You opened your eyes to another reset, and you immediately began rolling your plan around in your thoughts. The human wouldn't arrive until the next morning, so to make sure you catch them, you'd have to arrive early.
You'd act ill today, go to bed early for the energy, and the slip out before either of your brothers awoke.
The day went by in a blur, but as you returned home, you quickly put your plan in motion. You sat down on the couch, putting on your best weary expression and used your magic to make you warmer.
"(Y/n)? Are you alright?" Papyrus asked, frowning at your grunt of a reply.
You felt his hand on your forehead. "You're warm! Go on up to bed!"
"But—"
"No buts! Go on up to bed and rest!"
You sighed, giving in and heading up the stairs.
You knew if you didn't try to argue with him, he wouldn't believe you were sick.
Nonetheless, you turned in to bed, waking up to your quiet alarm hours later. You quietly creep out of bed, down the stairs, and out into the forest.
You waited several hours for the human to appear, but once those doors opened you were prepared.
The human looked surprised you were in front of them, but immediately pointed their blade at you.
"Not this time, brat." You growled, backstepping a bit as they ran at you, seconds before their chest was impaled multiple times. "Not this time."
Blood had splattered in your face, adrenaline coursing through you as you waited.
Waited for the reset.
Anger rolled through you.
"Why didn't you stop the first time? Was it not enough?" You stepped closer, staring into the human's dead eyes. "Have you finally seen your sins?"
"(Y-Y/n)?"
You turned, seeing Sans there, looking shocked and unnerved. You grinned at him, a slight hint of madness in your far too cheerful expression.
"Look Sans! I fixed it!"

(So y'all remember when I gave names to the various Reader AUs? Well this is Killertale's origin story kinda! Their name is Blood. :3 )