In this story, it is less an AU and more an alternate reality of the OG Undertale. In this, Chara has always hated the monsters and intentionally fell down the mountain to kill them all. She failed, and was left in the same state as the beginning of Undertale. When Frisk falls, determination awakens Chara, blah blah blah, Frisk is possesed, but Chara only kills Flowey, blah blah blah, read the rest. Stay with me on this. Plot might get a little complicated. But trust me, I knkw what im doing (I think).

Chara POV

--As you kill the flower that was once a long time ago your friend, an inaudible whisoer echoes through your mind. It gets louder the second time it sounds. The flower fades to dust. Now, on to the next freak. But first, let the new human get close to them. Then, kill them.--

--You have no memory of what just happened, and dont really care. Your back hurts-a lot. You see a silhouette coming from a dark portion of the plain you fell in.