After Frisk goes missing, Caitlin, Frisk's twin sister goes looking for her in the the last place she saw her. MT Ebott, the mountain of dark legends, she finds a necklace that belonged to Frisk on the ground. Just like Frisk, Caitlin tripped and fell into the Underground. Caitlin finds things she never knew existed.

"Promise me one thing- just one thing only." Frisk nodded vigorously. "No one can know about this whole reset thing. Not Papyrus, not Asgore, not anyone. It would basically bring this world to the point of falling to pieces at our feet. If we weren't trapped between time and space already, very few of us wouldn't be able to handle that knowledge, and could drive those who couldn't to the edge. So, please. I'm begging you. When you reach the End, when you get us out of here for the umpteenth time, I don't care if you reset or not. Just don't tell anyone about it, and... Please don't go homicidal again, okay?" Frisk smiled meekly, and nodded. "I can do that. And don't worry about the reset." They gave Sans a small hug. "After that whole thing with Chara, there won't be any more." And with the final word, they opened the door to leave; before exiting they paused. "See you later."

I'll take a crack at writing some Undertale. Please enjoy! (One shots and X readers)

You died. Again. But, when you woke up... You were back at the beginning. You didn't reset... but something was off. Flowey seemed skittish, frightened even. He was KIND. The people you knew, you once held dear... CHANGED. The entire underground was different... Dark. Nobody seemed to recognize you, but a part of them still remained. Someone who just wanted a friend, to be looked up to, respected... And another who just wanted hope-who was tired of this game... You're left with two choices: Will you kill to survive, or will you die befriending them all over again? Authors Note: This is in the AU Underfell. I recommend looking into it, it's simply amazing to me! Long story short, the "you" mentioned there is Frisk but it was a good description for the AU I found once. This is basically, somewhere the save file got corrupted, and now everything is reversed except Frisk. Hope you enjoy! Disclaimer: I do not own Undertale.