This contains a Headcanon birthed by refiinedrogue on tumblr and it was so messed up I loved it instantly. Thus this had to happen as I readily accept this headcanon

Also Flowey has like 15 parents, he stops himself from calling his actual dad "dad" but calls Frisk's mom "mom"

"Alright! I've narrowed it down to just a few choices!" Frisk exclaimed, setting a large cardboard box down in front of Flowey.

The small flower raised an eyebrow at the box before shifting his gaze up to the human. "Just a few, huh?"

"There were too many good choices for us!"

Flowey rolled his eyes, muttering 'humans' under his breath.

At his irritated nod, Frisk grinned and began pulling things out. The first was a black dress and red ribbon. "This would be for me – a young anime witch named Kiki and…" setting that aside she pulled out this ugly cat… thing with floppy paws, exactly Flowey's size. "…her familiar and best friend, Gigi!" Flowey's grimace of pure disgust was the only response. "It's pretty fitting to our –"

"No!"

Frisk set the catsuit aside. "Ok, how about this one?" She yanked out a red dress patterned with large white leaves, as well as a pair of large blue ears sewed to a small headband. She waved them happily. "We can be Lilo and Stitch."

Still scowling, Flowey tapped a vine thoughtfully on the ground. He hadn't agreed but he also hadn't turned it down.

"You get to solicit free candy from as many strangers as you want~," Frisk tempted.

Flowey perked up at that. A vine poked the box. "Got any I don't have to look like an idiot in?"

Frisk dug through the clothes and ears, setting aside a few choices that had Flowey pretty convinced he just might have to murder her if she had suggested them. Frisk finally sat back. "Well, there is one option where only I would be in a costume, and you'd be fine the way you are. But the fun is dress-"

"Sold!" A vine slamming the floor for emphasis. "I'm in it for the candy, not looking dumb."

…

Later that evening, as the sky darkened and street lights lit up, Frisk finished putting on her Pokémon trainer costume. The candy sacks had even been painted the red and white of Pokéballs. Flowey perched himself on Frisk's shoulder, and the two headed out.

The pair went to every house they came to – as long as the yard or house was decorated, or at the very least the porch lights were on.

Not a single monster took part in any part of Halloween: neither decorations or dressing up. Most wanted nothing to do with the event. For the most part Frisk understood: Sans and Papyrus had both been highly disturbed by any skeletal decorations or costumes meant to be frightening. The same went for anything ghost themed for Napstablook, Mettaton and MadDummy. Muffet had even put her bakery on a hiatus until all of this had passed despite how much money she would make for spider themed treats this time of year. In fact, Flowey was pretty much the only monster Frisk knew who didn't seem bothered by anything.

Sensing her growing curiosity, Flowey answered Frisk's unspoken question. "I only know as much as I could find in books or pry out of others. As far as I can tell, it has something to do with the first human-monster war. Something the humans did. No one liked talking about it: not in books and not in person. Even in all my resets I couldn't force the full story out of anyone." Flowey went silent, thinking, while snaking a vine into one of their candy sacks to sneak a piece out. "Maybe you can get it out of someone. D-, uh, Asgore wouldn't even show me the souls. ME. And he showed them to you in freaking minutes."

It was worth a shot…

"D'you mind pausing for us to go ask?" Frisk's only answer was the sound of Flowey crunching on something.

"Just leave the bags with me, I'll watch 'em," he said around a mouthful.

"Haha, nice try pal. I don't wanna spend the rest of the night cleaning up magic rainbow barf because you've over-eaten human candy." Flowey grumbled, but the thought kept him from insisting as Frisk made her way to the former king's surface home.

It didn't take many knocks before Asgore answered the door. As he did so, he was already mid-sentence explaining that he was sorry but that he was not taking part. He paused when he saw his visitors weren't just random children.

"Oh. Howdy Frisk, Flowey. What can I do for you?"

"Hi Asgore!" Frisk greeted first before adding, "I was wondering if it would be ok to ask you a few questions? They're about this time of year – you don't have to answer if you don't want to though!"

The tall old goat monster glanced back and forth between the two before finally sighing and standing aside, holding the door open. "Why don't the two of you come in."

Stepping inside and shutting the door behind her, Frisk set the two candy sacks down and followed Asgore into his living room. The warm room was rather cozy with large armchairs, a couch and a lit fireplace. It reminded Frisk a lot of Toriel's living room, although this one had more potted plants. There were also many family photos hung on every wall: Flowey purposely looked anywhere but at these.

Asgore took a seat in one of the large armchairs while Frisk opted to sit on the floor. By the looks of it, the two were in for a story and what better place to be when listening to a history lesson tale.

Running a large furry hand through his almost mane-like yellow hair, Asgore sighed again, carefully choosing his words.

"The 'decorations' people put up, while now they may be seen as such, weren't always playful decorations," was how Asgore chose to start. He had decided to cut to the chase rather than draw anything out. The two were children but they were clever. "A lot of times, especially in the olden days, it was common practice to hang casualties of the war out as warnings."

Frisk paled while Flowey stopped his noisy munching on candy. "There…there was something similar talked about in history lessons in school, but only related to human on human wars."

"There were not many wars before we were banished, young one, but I guess it is a method they continued to use after we had been removed."

Flowey chose to pipe up. "Monsters are made of dust though, how did they do anything like that?"

"Synthetic look-a-likes. Over time those turned into cheerful Halloween decorations."

No wonder none of the Monsters wanted anything to do with the holiday. Everywhere they looked were reminders of war, except now seen as fun.

Judging by the rather horrified expressions on Frisk and Flowey, Asgore added: "I apologize if this ruins your fun."

Frisk shook her head. "No, no it's ok. I mean I did ask." That being said, she would never be able to look at decorations the same way. "Was it… was it similar for costumes?"

"To an extent, yes, though those seemed to have developed more after we went underground."

After a while, Frisk and Flowey said goodnight to Asgore and started the walk back home. The pair didn't really talk much on the way, and there were far less kids trick or treating by then. After a while, Flowey broke the silence.

"We'll need to tell mom we must get rid of the decorations."

Frisk nodded in agreement.

"We can still get candy – and leave the porch light on for others to get candy but…"

She nodded again, adding, "Yeah, it'd be too weird setting things up again after knowing that."

It was very quiet; her footsteps were soft on the road. As they headed for home, Flowey snuck three more pieces of candy from the bag.