Preg-Dess encountered Mike and was changed by Mike in the dark world to be “better”.
Asgore doesn’t recognize her, sees Kris around her presence, and decides to try to kill her.
>He succeeds and it causes Dess to be a ghost in the dark void. And seals the door.
>When Dess sees what she looks like in the light world, she is a literal charred skeleton. She lives by the streets, crying.