more literally than you think
So yeah there's a bit of a background to this image so read the description because I won't answer questions that are answered in it.
So I've created my own sort of origin to Halloween Town and the holiday itself. A nameless creature from the underworld who bears no true shape, basically a living shadow, prowled the mortal world once a year to terrorize humanity into madness for his enjoyment. But then the first Skellington, a simple scarecrow, finally decided to step forward and stop the cruelty. He defeated the "Shadowman" or the original boogeyman, and created a holiday of that feared day, one that taught mortals that it wasn't a bad thing to be afraid and that it could be a day of terror but also fun and enjoyment. He was appointed by his fellow monsters as the "Pumpkin King" and Halloween was born. Completely insulted by what the Skellington had turned his holiday into, the Shadowman swore that he would take his holiday back and restore it to its so called former glory, and he would use a descendant of the Pumpkin Kings to do it as the ultimate revenge.
And with that little back story out of the way, you can probably guess which descendant the monster chose. Sadie Skellington showed a lot of potential with her magic so the Shadow staged an accident that would infuse a part of his essence with her, giving her an evil split personality that would try to destroy Halloween and everything the Pumpkin King held dear. After almost being overtaken by her evil half, Sadie managed to purge the shadow out of her, but by doing that she created a physical demon-like copy of herself that is now determined to take her place by any means necessary and carry out her dark purpose.