Into The Nightmare Before Christmas
Dawn stepped out into a courtyard where various monsters went about their everyday lives. A scary witch with a crooked nose greeted her with a cheerful screech and flew off on her broom while a ghost danced a jig with his ghostly crew. A skeleton in a pinstriped suit walked forward with a woman stitched together on his arm and he asked, "And who might you be?"
"I would be Dawn, who are you?"
"Welcome to Halloween Town, I'm Jack the Pumpkin King and the wonderful lady at my side is Sally." Jack smiled at Sally who smiled back.
Dawn nodded before looking around. "Do you know of a place I could stay for just a little while?"
"Why of course, you can stay with us!" he said with an evil grin and wide open arms, though Dawn knew he didn't really mean to look menacing but it just came to the skeleton naturally.
This was how Dawn became the apprentice to the scariest person in all of Halloween town. In an immortal town like Halloween town, Dawn didn't stick out as she barely aged and soon became friends of every resident of Halloween town. Over a hundred names made their way to her special notebook, each with something written under their name and picture.
As such a cute child, she didn't automatically look scary like everyone already living in Halloween town but her eerie toxic-green eyes and aura made everyone who looked at her nervous, she could simply give a smile and her eyes bore through the soul. She also learned hundreds of other scare tactics from various inhabitants which was also make easier because of her evaporation skill learned from the Cheshire Cat.
Dawn also spent some time with the very creepy Doctor Finklestein learning to stitch wounds and experiment which led to her experimenting on her own body. Dawn's first attempt left her with a scar on her chin where the scalpel had slipped in her hands and cut her. The experiment was successful, which was elongating her canines, even if she had a scar on her chin.
Her next experiment was much more dangerous and she couldn't perform it on herself so Doctor Finklestien did the actual procedure. She had made a design and changed all of her inner organs to work more efficiently and to stay more durable so she wouldn't have to suffer as much when she had lived with the Dursley's, which she could still remember in vivid detail.
Dawn stayed with Jack and Sally for another hundred years, learning all they could teach her before she was ready to move on to another world.