BtVS by Whedon and Mutant Enemy. Order of the Stick by Rich Burlew. I don't own the shows and comics mentioned in passing.
Written for Methos' "Tales from the Barman" over at Twisting the Hellmouth. The original story / set of stories, found at TtHFanfic dot org slash Story-11757 has over a hundred contributing authors. It's a varied setting, the main constants being that Xander Harris has opened a bar, called Nights, on the Cleveland Hellmouth. The bar is a weirdness magnet, with dimensional portals in the closet, and numerous characters (including Jackie) walking in, telling their story over food and drink, then leaving a memento for the wall.
Here's my corner of it.
Dawn Summers was sitting at the bar typing away on her laptop. She'd taken it on herself to catalog the objects decorating the back wall, paying special attention to those from alternate universes and potential futures. She was slightly worried that if no one paid attention to the stuff, especially with the field meant to keep the 'normals' from 'seeing' the assortment, the items would become less 'fixed'. Plus, she was bored and this was at least interesting.
"Hey, so you're running a trace on that phone number," Buffy said, peering over her sister's shoulder at the enlargement of a photo and the message scrawled beneath. Jackie Estacado, the current bearer of the Darkness, had wandered through in need of a friendly ear after finding out the downsides of his heritage. "That's the Catch-22 guy, right? Power to do anything - Death if he fathers a kid..."
The front door to the bar swung open.
"Lord Xykon, maybe we shouldn't go in. I sense anti-violence wards and anchored Detect Alignment spells," a voice carried into the bar.
"So? I just want them to fill a large order of drinks," came an oddly resonate reply.
"Yes, but maybe they won't serve us if they know we're evil."
Dawn and Buffy didn't even turn around, they'd heard much stranger things before.
"Oh, my God!" Dawn squealed, suddenly typing away furiously, pulling up blurred security cam footage from the files of another group of Watchers. "That's it! Xander said Jackie liked blondes, right? We could fix the guy up with Amanda! She's a thief and an Immortal. She'd walk away from almost any fight she got into to, he wouldn't have spend all his time protecting her and that Quickening means she can't have kids!"
"Hey, you're right, any female Immortal might answer his problem..." Buffy mused. "Hey, Xander? Xander, oh..."
Xander was watching the front door of the bar intently.
Many of the customers were.
Standing just outside the open door was a bleached skeleton with burning red eyes. He was wearing magical robes, probably marking him as a lich, or some other sort of magical undead.
Arguing with him was a short green and pointy eared demon in a red cloak.
Behind them was a tall being cloaked completely in darkness.
The darkness was being provided by a large pink umbrella. It had pictures of vampire cats evenly spaced around the rim...
"I know," stated the lich, apparently oblivious to his audience. "We can send him in. That umbrella protects you from that spell, right? Here, take the list and don't screw it up."
The large figure scooted in through the doorway, its exact shape obscured by the magical darkness. Only its bright yellow eyes could be seen. Worse, the wards didn't even breathe a whisper as it entered...
Two cat-sized red demonic roaches followed in the creature's wake.
The dark shape appeared to take a seat at a barstool and began to speak: "Barkeep, I'd like to order some drinks."
"Okay," Xander drawled.
The creature unfolded a list and began to read. "One Diet Lemon Iced Tea, One Gorgon's Blood, Thirty Angel's Tears, One Barrel of Rum Spiked with Mercury, Seventy Tankards of Mashed Brains, Oh dear..."
The bottom of the list unraveled and began to roll across the floor of the bar. "Seems each of the grunts wanted to list their own drinks and their writing is horrible... It looks like most of them ordered grog, but..."
As Xander stared open-mouthed at the length of the list, his hands twitching slightly, Dawn had the presence of mind to take a picture with the camera built into her laptop. She started running a websearch against the images of the lich, the goblin and the shadowed figure.
"...And I think we'd all like free pretzels, except for the zombies. Do you have a pretzel that's in scale with the barrel? We..."
At this, Xander firmly pressed a button hidden underneath the counter. It was the 'non-emergency' emergency button meaning the few vampire patrons of the bar had to an audible count of four to dive under the tables before the thick shutters that covered the bar's windows all opened at once, filling the large room with sunlight.
On the street outside, pressed up against the thick windows, was a literal army of short orange thugs, ghastly grey zombies, flying zombies with green wings...
The largest window was occupied by a zombified dragon with tarnished silver scales. It was staring directly at Xander like he was its next meal...
Screams went up from many seats.
The laptop beeped, saying the search was done.
Dawn tore her eyes away from the monstrous horde to look at the screen.
She silently rotated the computer to face Xander.
He looked down at the... webcomic...
"Andrew..." the one-eyed veteran Scooby bartender growled at the figure cloaked in darkness.
"Okay, who's the joker that ordered the Vampire Ashes in Holy Water? That stuff's flammable and I... Wait, what did you just say?"
"Andrew," Xander snarled, somehow showing each and every one of his kills on his face, in his glare.
Somewhat oblivious to the menace, the dark creature appeared to look over its shoulder, or at least its glowing eyes rotated around its body. "Hey Xykon? Does MITD stand for Andrew?"
"No, you idiot! It stands for Monster in the- Hey, look out!"
"That doesn't spell, hey - noooo!" the creature yelped as Xander reached across the counter to grab the Vampire Kitty umbrella by the handle.
A short struggle later, sped along by the help of the Slayers in the bar, the umbrella was wrenched away and the magical darkness disappeared, revealing the weak form of Andrew Wells sitting at the bar...
The horde outside and the lich and cloaked goblin at the door immediately vanished, so completely it was as if they'd never been there in the first place.
"But, but..." Andrew, the ex-member of the Trio and current attachment to the Slayer School, stuttered, before composing himself slightly. "What gave me away?"
"Look. I'd buy an interdimensional leak from a webcomic." Xander frowned. "Even a clever send-up of D&D. I'd accept a demonic horde pouring out of a weak area, say the broom closet. But a clearly story-based army appearing outside of the bar in the middle of a public street? To order drinks? Not gonna happen in Cleveland."
"Oh, well for bringing this epic story to your attention, not to mention the spectacle, could I donate my bespelled umbrella to the wall?"
"No, it's pink. And someone else's idea. How about the shopping list? You made that up on your own right? And something that many feet long..."
"No, I'm sorry, it was part of the illusion spell along with everything else. If you gave me a pen, maybe I could write it from memory... No, oh no, don't groan like that. You mean I went to all this work and I don't get to put anything on the wall? Is it my fault that messing up like that in front of Yoda when he came through the broom closet has left me slightly unhinged?"
"No, you don't. Yes it is. No, wait. Okay I understand what that'd do to your mental state. Complete geek to geek sympathy. But you need to realize that you're surrounded by pissed off Slayers you just traumatized..."
"Oh. Uh. Amnesty?"
Unnoticed in the ensuing tussle, one demon roach turned to the other and said: "I don't know about you, but I kinda feel real."
"Yeah, I don't think I'm illusion either. Let's hide before one of those superchicks steps on us."
Hours later the very battered Andrew was allowed to come down from his uncomfortable position on the wall.
Days later he was mostly forgiven. He was even allowed to host D&D games in a back room of the bar again.
It was around this time that Miranda, the bar's head chef, turned to Xander and asked: "Hey... Why are there two cockroaches, under the bar, fighting with lightsabers?"
He had one reply. "ANDREW!"