Upon repeated Genocide runs that lasted forever, Chara had decided to no longer give the Player control. And to hold unto their soul, resetting themselves. This time doing a run of their own without interference. Little did they know this would have unprecedented consequences on their world. Reality itself being adjusted, entities being thrown out of space and time, and those who were stuck inbetween, finding their way back.

https://www.youtube.com/watch?v=4OrUl3wtEzQ