Undertale: Twin sister

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.

Corruption (OLD)

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...