One Mushroom Kingdom day, on a completely ordinary-looking morning, that was totally NOT weird or unusual in any way, Mario was going through his normal morning routine: bashing Luigi's head with a pillow and whatever else he could find lying around, then heading to the bathroom so he'd look 'appropriate enough' to go outside. (He was pleased to find when he stepped on the scale that he'd only gained two pounds from last night's dinner.)
I could bore you with the details, but you probably just want to get on with the point. Luigi was still out cold, so when Mario was done he decided to check the mail. When he stepped outside, he noticed that the sky was dark and it looked almost about to rain.
That's weird, he thought. I could've sworn it was bright a minute ago.
Mario, not one to question stuff, started for the mailbox. When he opened it, a swarm of crows exploded out at him and ruined his just-styled mustache.
"Oh, come ON!" He yelled at the retreating birds. With a groan, he turned to the mailbox. There was just one thing inside: an ancient looking envelope. Unimpressed, Mario picked it up.
"What is this, a message from the 1800's?" As he opened it, however, he realized it was quite new. It read:

Dear mortal,

Congratulations! You're invited to a ridiculously awesome get-together tomorrow night, Friday the 13th, at the old mansion outside Toadwood Forest. Put aside the differences as we have our own Nintendo-style house party courtesy of King Boo! Free money and a latte-sipping contest will be included. Don't hesitate to come on down and have an unforgettable time...

Sincerely,

King Boo's goons

(P.S. Don't forget to bring your brother)
(P.P.S. Nice mustache, chump)
(^that's funny right there)

Mario finished reading and squinted at it. Before he could even think Luigi was on top of his heels.
"What's that Mario?"
Mario jumped. "Luigi! Weren't you just in bed a second ago?"
Luigi grabbed the paper. He gasped. "A house party!...At King Boo's!"
"Yeah, pretty rad, right?" Mario asked. "I'm totally going. All our friends are probably gonna be there."
Luigi blinked. "Well...but...don't you suspect anything?"
"Why should I?" Mario asked, unconcerned. "It's just for fun, I'm sure. Be real, Luigi."
"B-but...I don't know, something just bothers me about getting an ancient looking letter inviting us to a random spooky-sounding place for a party! I know better than to trust mail," Luigi said. "There is evil in it that does not sleep."
"Oh, don't worry, Luigi," Mario said dismissively, heading back inside. "I'll go alone. You can stay here."
"Mario are you crazy?! Going to King Boo's by yourself on invitation? Hello! That is not happening again!" Luigi cried.
"Luigi, there's free money and a latte sipping contest! How can you expect me not to go?"
"Mario, have you forgotten everything about King Boo? I'd like you to come home, please! He's...evil," he whispered, glancing around as if someone was watching.
Mario rolled his eyes. "Only sometimes. Think of it like a Mario party, Luigi. I'll be fine." He stopped, then added, "But you should stay home and lock the doors."
Luigi stared at him in terror. "No Mario! Don't go! At least don't leave me!"
Mario groaned. "Alright, you can come with me if you really insist," he said. "Just don't nag me."
Luigi crossed his arms. "If someone invited you to a friendly heart transplant testing session, would you go?"
Mario raised an eyebrow. "Why do you ask?"
Luigi shrugged and grinned slyly, heading past him. "No reason."
Mario frowned after him as Luigi disappeared into the house. "I am not gullible!" He shouted.
"Whatever. Nice mustache, Einstein," Luigi snorted.
Mario groaned and turned to the audience. "This is a weird story."

Meanwhile, on that same dreary afternoon,  a similar letter fell at the castle's gates for Princess Peach. In the jungle, Donkey Kong also received a similar summons, except his was written like this:

Ook ook,

Ook! Ook ook ook ook ookity ook ook, ook ook, ook ook ook oook ook. Ook ookity ook ook ook ook ook! Ook ook ook ookity ook ook. Ook ook ookity ook ook...

Ookity,

(Cackle cackle snarf)

To which his response was: "OOK! Diddy, we're gonna get some free lattes!"
"Lattes?! Ooh, we're gonna have to sing our latte song!"

NintendoJedi: NO singing!

As you can understand, it was a very convincing letter.
The goons also received this letter and each thought it was some sort of prank by the other; only after getting together and slapping each other silly in the faces did they realize it was 'obviously legit', to put it in Waluigi's words.
Waluigi squinted over the edge of the paper and smiled evilly. "Wario...we're gonna get some lattes."
When Princess Daisy got an invitation, she promptly called Peach. "Hey, did you get one of these invitations?"
"I sure did," Peach replied. "What do you think? Sounds fun!"
"Oh, sure. A little weird, maybe, but no way am I gonna miss a party!"
It may not come as a surprise that a letter even found its way all the way to Dark Land, on Bowser's doorstep, to be exact. When the mailkoopa brought it he found a lovely mailbox in the shape of Bowser's head. A sign below it pointed to the open mouth and read 'Feed Me'. Weirded out, he dropped the letter on the ground and left. Ludwig found it when he came out to booby trap the driveway later.
"Hey, boss! I think you got a love letter!" He said, waving it around as he went inside.
"From Peach?" Bowser Jr. asked hopefully.
"No," Bowser growled as he opened it. His facial expression changed many times before he reached the end. Finally he laughed and said, "Oh, that's diabolical."
"What?" The koopalings asked excitedly.
"Junior, get on your game face," Bowser announced. "You and I are going to a little...friendly gathering."
And so it happened that all of our Mushroom Kingdom people ended up planning to go to the party. But far was it from any of their minds what was really in store for the following night...

And wouldn't you know, we get to skip ahead.
Following night!
The evening was dark and dreary. Wind slashed at windows and owls sang sick songs out in the gloom as everyone was getting ready for the party that night. In Mario and Luigi's house, Luigi was standing in the hall alone, banging on the bathroom door like he had been for the past ten minutes.
"Mario! Hurry up! What could you possibly be doing in there?"
"Stuff! Why?"
"Cause I have to get ready too!"
Mario opened the door, looking as normal as ever. "Really? You don't want to stay and keep an eye out for burglars?"
"Uh..."
"Or monsters or ghosts that knock on the windows, trying to....oh, I don't know, eat your soul?"
Luigi looked around nervously. "Stop it! I'm coming with you," he said, shoving him aside. "And honestly, how much mustache wax do you really need?"
"It's not that much more than usual," Mario said.
Luigi coughed. "And sheesh, what are you using, like an ounce of cologne?"
"It's a refined smell."
Luigi groaned and turned away. "If that smell doesn't kill the entire army of Bowser, I don't know what will. What's it called?"
Mario shrugged. "Boo venom."
Luigi widened his eyes. "I don't know what's going on, but I have a feeling I am really not going to like this story."

At the castle, Peach and a few toads were standing outside the gates, just about to leave. "Toadsworth is in charge while I'm gone," she instructed. "Don't touch the knives, don't open any windows, and have fun!"
"You too, princess!" Toadsworth waved. "Do try to be back before 3 am! And PLEASE be careful!"
When they were out of sight, he turned to the toad beside him. "I don't know, I just get a very odd feeling. What's today's date?"
"The 13th, sir," the toad replied.
Toadsworth froze. Then he fainted into the toad's arms.

The full moon glowed faintly through the clouds over Toadwood Forest as Mario and Luigi approached the old house. They had gotten temporarily lost on the way, which had freaked Luigi out profusely, but Mario quickly got them on track. Unfortunately, the sight of the building didn't exactly make him feel better.
"Well, here we are," Mario said, climbing onto the porch steps. The place looked old and abandoned, possibly a good hangout for crimelords, but glowing lights and the faint sound of music came from within. Jack-o-lanterns with weird stuff like boos and creepy eyes on them were scattered all over the lawn.
Luigi shivered. "Why would anyone hold a party here?"
"This is King Boo we're talking about," Mario reminded him. "It's his style."
He used the skull knocker without batting an eye. "I hope we weren't supposed to dress all fancy."
Luigi carefully slunk up onto the porch. "Are you sure this is a good idea? It's not too late to turn around," he pointed. "We can still leave."
"Why would I want to do that?" Mario asked. Just then the door opened and a boo in a black bow tie greeted them.
Luigi groaned. "Too late."
"Ah, Mario and Luigi," he said. "The boss has been expecting you. Please, come in."
He led them through a dimly-lit hallway that looked like it hadn't been cleaned in fifty years to the big gathering room.

Here play this King Booish disco:

[There should be a GIF or video here. Update the app now to see it.]

Some sort of spooky arrangement of techno music was blaring from unseen speakers, accompanied by rainbow disco lights that were enough to give any normal person a seizure. Luckily, no one here was a normal person.
"The party don't start till I walk in," Mario said to no one in particular. The rest of the guests, who were already there, looked up at their entrance. They were surprised to see Princess Peach, Daisy, a few toads, and Rosalina standing around a punch table chatting. Donkey Kong and Diddy Kong were goofing around in a corner with some ginormous spiderwebs, seemingly oblivious to the fact that they were partying in a glorified graveyard. Yoshi and Birdo were there with some koopas and shy guys, and even Bowser and Bowser Jr. were there, talking to King Boo nearby, who was chilling on a big beanbag throne. Everybody froze when they walked in.
"Well, hey there, guys," King Boo said, holding up a latte. "Glad you could make it!"
"Yeah," Bowser chimed in. "Welcome to King Boo Party!"
"Well, thanks for having us," Mario replied. "I assumed it might have been a villains-only party."
"Nah, not this time," Bowser said, stirring his latte. "That would have been rad, but the guys at Nintendo basically told King Boo that he had to invite all you goons."
"Right," King Boo agreed. "No hard feelings. Mi casa is tu casa...or whatever."
Luigi frowned. "Hey, isn't that what you said -"
Mario turned to Luigi. "See? Nothing to worry about." Then he made a beeline for the food table to chat with the girls.
Luigi looked at the baddies nervously. "You're completely missing the point," he grumbled.
"Oh, Mario! So good to see you!" Peach greeted him at the desserts.
"Glad you guys could make it," Daisy spoke up. "Even Rosalina here!"
"Well, the Lumas thought it'd be nice if I could join more of the Nintendo parties and functions," she said, sipping her wine glass. "But either I'm very uncultured, or this is a rather old place to be having a party."
"Well, it's probably the size," Daisy said. "King Boo doesn't have a castle."
"But he has plenty of mansions!" Mario said. He could feel Luigi's glare from the doorway and quickly fell silent.
"Okay, everybody!" King Boo announced, bringing the music to a stop. He tried to clap his hands, but when he looked down he realized his stubs didn't reach.
"Now that everyone's here we can get started! Wait...we're missing two people," he said, scanning the room.
"Did you invite Professor Gadd?" Luigi asked timidly.
King Boo snorted. "No. The old coot is too busy inventing stuff to even spare the time and frankly, I wouldn't stoop so low as to bring him here."
"Then why'd you invite me?" Luigi challenged.
King Boo gave him an unsettling smile. "Why, because you're one of my regulars!" He said. "Now if you'll excuse me for a minute, feel free to mingle until I find our missing guests." Then he floated from the room.
Blue Toad stood on top of an old chair. "THE HOST IS GONE! EVERYBODY PARTY!" he hollered.
Yellow Toad, already in position, promptly blasted the music so loud the princesses' glasses of drink shattered. Luigi could have sworn he felt the floor and walls trembling.
Mario took that as his cue to raid the food table. His feast was interrupted by no one. After that on his list was the punch table, then the snack table, then the dessert table...
Yoshi and the others flooded the dance floor with over-the-top dance moves that probably were inspired by Michael Jackson. This finally convinced even Luigi to join them, who actually busted out a few wicked breakdance moves before someone squirted some mustard on the floor and he slipped in it and some koopas laughed at him until they slipped in it. And for about three minutes, it was party perfection.
Then Birdo, who had apparently snuck behind the stereo, cranked up a swift-paced, dramatic dancing song. Mario promptly took up his hat and turned to Peach with a flourish. "Princess, may I'm-a have this dance?"
Peach gasped. "Oh, of course, Mario!" They joined hands a little awkwardly and hit the floor. The toads and koopas and all the eggs Yoshi just laid joined them until it looked like some ridiculous throwback to Snow White. Occasionally one of them would jump too hard and fall right through the floor.
Luigi was standing there awkwardly (I'll let you know if he ever stands there not awkwardly) until Daisy grabbed his collar and pulled him out into the floor.
Then things seemed to move well. Yoshi and Birdo joined each other for the couples' dance, Bowser Jr. started hiding under tablecloths and grabbing peoples' ankles when they came for food, causing them to shriek like hyenas, and the shy guys were just doing... their own thing, which basically looked like screaming in each other's faces. Boos were floating around like glowing lanterns, bobbing to the music, except for the ones literally bouncing off walls like they'd just charged up on Red Bull. Only Bowser stood there watching and growling to himself, probably plotting to poison Mario's food or something.

Bowser: Hey! You're not allowed to broadcast everything I'm plotting!

NintendoJedi: Oh yes I can!

The toads catapulted themselves from giant spiderwebs and Yellow Toad ended up breaking another hole in the wall. Plus three different koopas and shy guys almost had heart attacks due to the fake hand in the candy bowl. And that's pretty much the low down of how the party went for ten minutes.
When the song ended, some people broke up and fanned out, making things a little awkward. Peach and Daisy regrouped with Rosalina.
"Well, that was lovely!" Rosalina told them. "You certainly know how to have a splendid party."
Peach almost blushed. "Oh, thank you. It wasn't me..."
Daisy laughed. "Yeah, even when it gets out of hand, I still love it."
When a rather disoriented Luigi made his way back to Mario again, he found him talking with Bowser by the snacks.
"Heyyy, Mario, short time much see," Bowser said, sidling up to Mario's side. "And by that I mean, why won't you get out of my life for a while?!"
Mario turned to him, unamused. "Bowser. Why won't you get out of my punch?"
That would have been where he'd fling his whole cup of punch in Bowser's face, but before he could there was a sudden loud sound, like a siren.
Luigi screamed. "Cover your ears! Don't listen to it!!"

NintendoJedi: Um, Luigi, it's not that kind of siren.

Luigi took his hands off his ears.
"WAAAAAAAAAAHHHHHH!"
Without warning the chandelier broke from the ceiling and came crashing down right on top of the impressive nacho display. It brought a couple of goons with it. Because honestly, we all knew that's what this party was missing.
"Waluigi has entered the building!" Waluigi announced from the snack table. "And quite stylishly, I might add."
Daisy looked up. "Well, it was fun while it lasted."
Wario jumped down, flinging sauce from his clothes. "Did you miss us?"
Mario and Bowser, standing there covered in various nacho toppings, looked ready to beat on someone. And for once it wasn't each other.

When King Boo returned to the hall a second later, he found a... startlingly different display from what he'd left about ten minutes earlier. His missing guests were chasing each other all over the room with nachos. Toads were spinning each other all over the floor and busting open cans of soda and bags of chips, flinging it everywhere. By now the boos had gone wild and were bonking each other with balloons and every other hard object they could find. Bowser Jr. and Diddy Kong were messing with the turntables, and now had managed to turn on the DK Rap, forcing everyone but Donkey Kong off the floor.
"Hey! No messing with the turntables!" a boo yelled at them, but no one heard him because it was too loud.
Mario and Bowser were casually leaning against a wall, having come momentarily to terms.
"So Bowser, how's the secret wife?"
Bowser did a double take and threatened Mario with a fist. "No one ever said I have a wife!"
"Well the existence of Bowser Jr. kinda says you do."
Bowser Jr. looked up. "Fat man says what?"
"Hey, Junior, why don't you tell Mario about the time you went hot tubbing with Princess Peach?" Bowser suggested.
"What?!" Mario said.
"Ex-cuse me, everybody," King Boo interrupted. "I turn around for five minutes and you've all wrecked the place? You don't know what kind of mortgage I have!"
"This place is actually worth money?" Waluigi asked incredulously. "It looks like it could cave in if Mario jumped."
"Don't tempt me, nacho boy," Mario warned.
"Oh, that's how evil parties always go," Bowser said. "Just let people do their own bonkers things."
"Well... Now that everyone really is here, how's about we get this party started?" King Boo suggested.
"Nice, but we already did that," Mario said. "There's not much to a party except crazy  wild music and crazy wild dance moves and crazy wild food, although I always said a party's not a party until someone gets injured or Luigi cries."
Luigi opened his mouth and glared at him.
"What exactly is 'wild food'?" Wario asked.
King Boo ignored him. "Well how about some free money?"
"YEAH! Gimme the money!" Wario demanded. "That's the only reason I even came to this dump!"
"Ha," King Boo snorted. "Well, I hate to be the pooper of your parade, but unfortunately, I'm afraid there is no money to give you at the moment."
"HUH?!"
Everyone looked at each other as if they'd been duped.
"We've been duped?!" Waluigi screamed.
King Boo continued grinning mischievously. "I didn't say there was no money. Just that I don't have it."
"Um, yeah he did," Daisy whispered to Peach and Rosalina.
"What kind of joke are you pulling?" Donkey Kong asked.
"Yeah!" Mario said, a little angry. "My invitation promised free money to the last letter, and that's what I expect to get!"
"It's no joke, you nincompoops! I'm saying, you're gonna have to work together and find the money! It's hidden somewhere in this house. You know, a game?"
Everyone looked at each other. "Where is it?" Rosalina asked.
"Ugh. That's why it's called a treasure hunt," Bowser snorted. "You have to hunt for it."
"I had to give you fools something to do besides wreck the gathering room and blast awful music. I may not have ears, but but you can still go easy on the hearing."
"So... a treasure hunt, huh?" Daisy asked. "What do we do?"
"You look for the treasure in a room that spins," King Boo said. "Twists and turns and time will tell. I'll be hanging out here being rad and stuff while y'all look. The mansion has a few floors...and look out for holes."
Everybody looked at each other.
A koopa nearby groaned. "Okay, all secondary characters come with me," he said, leading them out. King Boo followed them but turned back at the last moment.
"Oh yeah, and one more thing. If you guys don't find it by dawn...there'll be no coming back."
"Uh...what does that mean?" Peach asked.
"Oh no, it's a scary riddle," Toad quivered, hiding behind Peach's skirt. "I was so afraid there'd be a riddle."
Luigi's eyes grew wider than Bowser, pre-diet.

Bowser: Hey!

"What if we don't care about money?" Luigi asked.
Wario promptly slapped him across the face. "Shut it, greenie! He didn't say that."
Mario turned to him. "Hey! Only I'm allowed to hit Luigi like that!"
King Boo floated to the doorway. "Well, have fun. And shut off that stupefying music. Later!" And he slammed the door shut.

Twenty two seconds later, Luigi shivered as he leaned against the wall, trying hard not to hyperventilate. The whole group stood out in a hallway, trying to decide what to do.
"Great. Just great," Waluigi said. "I come to this party and basically end up locked in a nuthouse with all of you hooligans."
"You're one to talk," Daisy said.
"Oh, I don't know if this was a good idea," Toad said worriedly. "What if it was all just some trick to trap us here, so King Boo can have us for dinner later?"
"Nah," Mario said. "We just need to put our heads together and find the treasure! Luigi, quit chattering your teeth like that," he added. "Sheesh, I can't hear myself think."
"But-
"No butts! Let's get moving!"
"I hate to say that for once, I agree with him," Wario said. "We need to find the treasure. Look people, I didn't come to this party to socialize. I didn't even come for the free food."
"No one is interested in hearing about your selfishness right now," DK broke in.
Mario interrupted by burping. Everyone looked at him.
"Excuse me. Man, I'm-a just so hungry..."
"How could you possibly be hungry?" Waluigi asked. "You spent the first five minutes stuffing your face!"
"Yeah, but it's already been five minutes since then."
Just then Waluigi gasped. "Wait, did they have those awesome little wieners that look adorable?"
"Yeah," Mario said, rubbing his stomach, "but I ate them all."
"How could you?!"
"Look, it wasn't personal. I ate all of everything."
"EVERYTHING?!" Several of them said in unison.
"I could be incorrect, but that seems physically impossible," Rosalina said.
Luigi shook his head. "It would be."
Waluigi snorted. "Great. Now we're stuck in here with you goons and no food."
"Hey, why don't we go that way?" Yoshi asked, pointing down one end of the hall.
"Looks like a perfectly logical choice."
"As opposed to that way, which is a perfectly illogical choice," Bowser pointed in the opposite direction.
"It doesn't matter! Let's just get this pasta on the fork," Mario said.
Everyone looked at him. Then Bowser said, "I'd rather not work with you goons unless I have to, so if you'll excuse me, Junior and I are going this way." He turned around and started down the hall.
"Okay, it's probably your funeral!" Birdo yelled after them.
Just then Luigi gasped. "Wh-what was that?"
"What?" Birdo asked.
"Didn't you hear that thump?"
"It's probably just the boos playing hopscotch," Toad said.
"No, I swear I heard something else!"
"It's just your jitters showing," Mario said. "Come on, I ain't afraid of no ghosts."
"Yeah, that's your issue," Luigi grumbled.
"Well, I agree with King Mutant Turtle over there, so we'll catch you losers later," Wario said, heading after Bowser with Waluigi. "Good luck getting lost."
So, with their now reduced party, the gang ventured forth. (Cue epic walking down hallway music.) After about ten seconds they hit a door.
"It's locked," Mario reported.
Everyone groaned.
"Wait, let me try it," Luigi said.
"Why?"
"'Cause maybe I have the finesse!"
"Luigi, the only thing you have 'finesse' at is finding the perfect sleeping position."
Luigi gasped. "That is not true!"
He grabbed the doorknob and pulled. When that didn't work he put a foot on the door and strangled it like madman. Finally he stopped and turned to them, out of breath. "It's locked."
So they turned around and headed back the way they came. (Cue backwards epic walking down hallway music.) When they passed another door, Mario stopped.
"Wait, isn't this the one back to the front?"
"I think," Daisy said.
"Maybe we should ask King Boo for a direction," Peach suggested.
Mario tried the doorknob. "It's locked."
"This can't be the same door," Rosalina said.
"But I swear this is the same spot!"
"Well it can't be," Blue Toad said. "Unless..."
Luigi stepped forward. "Mario, I don't like this mansion. Something feels weird."
Mario rolled his eyes. "I wonder why."
"Because these doors are playing tricks on us, and you're acting dumber than usual. I'm telling you something's not right!"
Birdo groaned. "What on earth are we supposed to do?"
"Alright people, let's think," Mario said. "Don't lose your heads."
Then a harsh crack of lightning promptly plunged the place into blackness.
Half the people screamed. When the lights flickered a moment later, Luigi was alone in the hall. He screamed again.
He turned to the door that was supposed to be beside him...and it was gone.

Dun dun dun...
And so we come to our first ever cliffhanger. I know I had been trying to keep these stories one part only, but this one would have gotten too long if I didn't split it. And once again, it took me entirely too long in the first place. I wanted to do something for Halloween...and so this is it. The conclusion will come AS SOON AS HUMANLY POSSIBLE
That song up there was actually the main inspiration for the story. I heard it and thought it really sounded like some sort of King Boo Party...so this happened. There are obviously a lot of Luigi's Mansion references...and the hot tub thing is a nod at Sunshine. Toadwood Forest is actually a thing from Mario and Luigi: Partners in Time. It just seemed very fitting for this story.
And for those of you who have not heard the DK Rap, I will now either make or ruin your day by playing it here:

[There should be a GIF or video here. Update the app now to see it.]