So this is one of my first times venturing into the crossover and really some of my first fan fic work. But I have been waiting way too long to see this actually happen in the show, so I'm just going to do it myself. Let me know what you think!
Disclaimer: I do not own Once Upon a Time or Nightmare Before Christmas :/
Enjoy!

A lone figure shuffled through the darkest part of the woods on a Sunday night. Wind softly blew her brown hair out of her face as she wrung her hands in worry and kept walking. Her legs ached and her feet were so sort they felt like they would fall off, but she kept on walking. She wasn't sure where she was or where she was going. Still, she had to be going somewhere. Nothing looked familiar, which might have excited her at one point. Jack always liked to see or hear about new places and she would relish telling him about the silvery moonlight spilling through the trees or the way golden leaves crackled underneath her feet with every step.
Sally glanced down at her hands again, reassuring herself that they were a human color. All the residents of Halloween Town knew how to disguise themselves in the human realm, the just seldom had the need. She was unused to the look of a pale, fleshy hand reflecting in the starlight.
"Stop right there." A hard woman's voice ordered from her right. Sally turned to face a blonde in a red leather jacket.
"Oh, I told Jack this wasn't going to end well. I've told him a million times, but he just never listens. And just a month before Halloween, that fool!" She twisted her hands together some more, giving a worried look to the rather confused looking blonde.
"I'm sorry, I'm just going on to myself. I'm Sally, and I'm afraid I'm terribly lost. Could you tell me where I am?"
The other woman who had at first appeared to be in a position to attack, relaxed. "StoryBrooke. Now how did you get in here? Nobody gets past the town line." Her posture said she was relaxed. Her words and her tone told Sally that she was still poised and ready to fight.
"I'm not really sure. One minute I was getting ready to get married, the next I woke up in these lovely woods. I've been wandering for a while, hoping that I would find someone." She explained, offering a soft smile in attempts to calm the other woman. "What's your name?"
The other woman stared for another moment, face still hard. Then she tucked the gun in her hand into the back of her pants and seemed to settle for eyeing Sally warily. "Emma. Come on, I'll take you into town, we'll get you on your way back home by the time morning comes." She said sharply, waving Sally forward. Sally hurried after Emma, tugging at her full black skirt and grimacing when she heard a rip. She had worked for weeks on making the wedding dress, she didn't want it ripped to shreds by the time she finally saw Jack. Still, Emma charged through the woods as someone that obviously knew them well and didn't intend to slow her pace for Sally. She had no choice but to hike up her skirts as much as she could and do her best to keep up.
It didn't take too long for them to come to the edge of the woods. They stepped out onto a road where the oddest looking yellow car waited. Sally's hand timidly flitted out to touch the side of it. "How lovely." She said with a happy smile, looking over to the other side of the car where Emma was giving her a slightly confused and impatient look.
"Thanks... It's my bug. Get in, I'll drive you to the station. The dwar- Some hikers said they saw someone out in the woods, so I'll have to let them know that I found you."
Sally slipped into the car without a reply. She could tell that Emma was hiding something, she had really done some tripping over words, barely keeping from saying something. Sally was beginning to have a terrible feeling in the pit of her stomach about the strange town, remembering the odd premonition she'd had that morning.

Sally swept the comb through her hair once more, smiling at the way her brown hair was curling over her shoulders. She wore a tiara made entirely of spiders on her head and her dress was a lacy black material that the witches had spent hours creating. The result was breathtaking and Sally could hardly believe that she was wearing it. Still, it was nothing compared to the fact that she was getting ready to go marry Jack. That was a wonderful fact that she had yet to wrap her head around.
She gazed into the mirror in front of her, allowing her mind to wander. Perhaps one day they would have a few skeleton children running around. Sally giggled a little at the thought, absently noticing the picture of a town forming in the mirror. She often had visions of strange places, so she didn't pay it too much mind. Until flames began to lick at the edges of the buildings. Soon, the whole town was engulfed in flames and screams echoed in Sally's mind. She gasped and pushed herself back from the dressing table. She had to go find Jack.
Sally hurried from the back room in the town hall, ignoring the sound of ripping fabric as she carelessly tore through the doorway. She raced to Jack's house, a tower that reached for the sky, and pushed her way inside without knocking.
"Jack! Oh Jack!" She called, starting up the stairs. He appeared at the top before she was halfway up and met her in the middle.
"Sally! You look absolutely stunning, I fall in love with you more every day." He took both of her hands, looking at her in that way that always made her feel all muddle-brained inside. She almost forgot all about why she had come. But the picture of a burning town popped into her head again and she straightened up.
"Jack, I had the most awful vision" She panted. He smiled.
"How delightful!"
Sally shook her head. "No, it truly was dreadful! There was a town, and it burned up. So many people were hurt, I could hear them screaming." She insisted, tugging on his hands. He had to grasp the importance of it, especially after what had happened over Christmas.
The skeletal man just shook his head. "On our wedding day? Not a chance, nothing like that is going to happen. I won't allow it." He said confidently. Sally couldn't fight back the worry as easily.
"But what if you can't stop it? Or what if we're supposed to, and we do nothing and it still happens? What if it's a sign-"
"Stop worrying, Sally." Jack cut her off. "Everything is going to be fine. I'll have the mayor check the town to make sure everything is in order, and we shall assume that this disaster will hold off another day." He assured her, cutting off any further objections with a lingering kiss.