The CORE has shut down. Power to the underground has been depleted for weeks. The population is starting to grow mad. With no way to cook food, the monsters are close to turning on each other for survival... Asgore has left, possibly forever for his grief and disappointment of himself of not being able to save his kingdom in this trying time. Undyne takes the crown instead. The new Warrior Queen of the Underground. You were one of the residents, aiding Sans and Papyrus in supplying of rations to the people. But as time passes, you know they were starting to run low... It's only a matter of time. So Sans goes off to get some more from the palace. Saying "I'll be back soon to help the others!"... That was a few hours ago... Something's definitely fishy. Papyrus is worried sick... So you decide to investigate. And after a perilous journey to the palace. You arrive. You hide on the balcony, overhearing a conversation between Sans and Undyne

"You must understand, Sans... This is for the better of monsterkind"

"so? I'm not letting you touch my eye... end of story"

"Do you hear yourself? Putting yourself over everyone else? Why am I not surprised... Your the laziest one after all"

Seems like Sans gets offended by this

"And your calling me lazy even though I've spent the past 3 days supplying food to the people in Snowdin? Why you've sat on your ass, doing nothing?!"

"YOU DON'T KNOW WHAT ITS LIKE!"

Sans pauses, but then chuckles

"By Asgore... You really have lost it... All bark, but no bite... Maybe instead you should try helping us instead of sitting on your fishy ass like a bit-"

Before he could finish, Undyne punches Sans directly in the skull. An audible crack is heard as Undyne rips Sans' magic eye out

"You left me no choice... This is the key to saving us... All of us... And it only took 1 measly sacrifice..."

Undyne stomps off in her armor, probably towards the core. While Sans lays on the floor, not moving either from shock, or being knocked out. Your not entirely sure...