The smoke from the floor started billowing out like cheese whiz at a toxic waste factory.
"OHOHOHO!! Oh look, it's the milkman!" The eerie voice boomed from the floor. "Look guys, the milkman's finally here!"
Luigi perked up. "You believe me??"
"NO YOU NIMROD!!" It hollered, sending Luigi slamming back against the wall. "You're Luigi! Nobody's afraid of you, even if you were bonkers with a chainsaw!"
"Don't put it in his head," Mappy muttered, but nobody heard him.
"Who are you??" Luigi yelled. "How do you know who I am? And why are you in my storage hole??"
"BWAHAHA!! Your storage hole? Think again, fool! This is OUR mansion!!" All at once, a huge explosion of white stuff (not smoke) came spewing out of the hole. And what it was was
BOOS!

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

The earth was so horrified by all these boos that not even Mansion said anything, which is surprising at this point.
All at once, billions and billions of boos began erupting out of the floor hole like a geyser giving birth to boo babies —
Okay that was a bit much. There was only like 37 of them in there. But anyway.
Luigi screamed and sat against the wall like a scaredy baby. "AAAAAHHHHH!!"
In a matter of seconds the boos were everywhere, having filled the whole room with their roundish bodies and stubby tails. They were flying around all over the place, like rioting baseball fans who couldn't afford tickets to the World Series.
...Okay that one didn't even make sense moving on
"Bwahaha! Yes, scream, you babyish tiny little child man! Scream for your mommy who isn't going to come save you!"
Luigi gasped. "What did you do to my mommy??!"
The boos all seemed to pause in midair, which made the edgy techno music suddenly stop playing. One boo, who was actually Gameboo if you recall, stared down at him. "No, you fool! Don't you get it? This is a game!! And you're playing it!" It was like trying to find a specific boo at a boo convention, or something to that effect.
Anyway, Luigi glanced over and noticed LR watching all this from the mirror, but he didn't say anything.
"Whatever. We couldn't care less about the woman that birthed you, or the business executive that greenlighted you, or the character designer who spawned you. All that matters is that you're here now... and you've fallen right into our brilliant little trap!"
"Trap? What trap??" Luigi asked genuinely.
A nondescript boo sighed. "The one that you're inside of right now in the middle of a dark suspicious forest instead of chilling in your armchair at home eating pot pies or something! Use your brain man! ...I can't believe you've lasted this long."
Luigi frowned. "Hey, are you insulting me? Because that was insulting!"
"You're begging for it, kid. And now..." All the boos suddenly seemed to turn on him in a nasty wave of beady glares. "We're going to do to you what we did to your brother... only worse."
There was an awkward silence as Luigi stood skeptically for a minute.
One or two of the boos blinked. Luigi shifted.
"Alright, which one of you is talking? Because I find it so annoying that I'm looking at all of you and I can't pinpoint where your voice is coming from."
A boo cleared its throat. "Excuse me, I said we're going to do to you what we did to your brother, only worse!! Why aren't you flipping out??"
Luigi paused again. "Am I to take that as proof that you had a hand in Mario's disappearance?"
"OH COME ON YOU IDIOT!! YES WE TOOK MARIO!! AND DO YOU WANNA KNOW WHAT WE DID WITH HIM??"
Luigi opened his mouth, then hesitated. "Um... yes. ...No, no don't tell me actually. ...Bend it a little?"
"Haha! As if we'd actually tell you! That's part of your punishment, dolt! You don't get to know!"
Luigi stood up, looking a bit more composed and upset. "Well, that's a real nice way to treat somebody who offers you free milk!" he said.
"WOULD YOU DROP THE MILKMAN THING!!" Mappy yelled.
There was a brief moment of awkward silence in the storage room.
"Was that voice from your pants?" Taboo asked.
"That's classified," Luigi replied. "And how do you know who I am, anyway? ...Am I that famous??" he squealed in excitement.
"Don't work yourself up, kid," Boolivia snickered. "The main reason you're here is because the boss put a price on your head."
"But, I'm famous," Luigi prompted.
Mappy groaned in his pocket.
"You're a close relation of Mario, and that makes you a jerk to boo-kind," Boodacious put in. "So that means we hate you."
"ALL of you? Collectively?...And I thought I had issues," Luigi muttered. "Speaking of, are you trying to tell me that I didn't win this Mansion?? That it's all a mean joke??"
"That's exactly what we—
"HA! Shows how much you know! I have the papers on it right HERE!" Luigi said, pulling them out. "This property is mine, fair and square. So just... back off, buddy boys."
The boos, very weirded out and fed up already, rolled some of their eyes. "Fantastic. Well, listen, we have an operation we're running here, so we'd appreciate if you made like your brother whom we already captured and came with us unconditionally."
Luigi frowned thoughtfully so you could almost see the gears in his head turning. "You captured Mario? Yeah, I dunno about that..."
"Well we did," said Underboo.
Luigi shook his head skeptically. "Ehh. Prove it."
The boos looked taken aback. "Wha-? We don't have to prove it! Our evil word is good enough!"
"Hmm... well, come to think of it, those paintings called Mmmm did mention something about boos," Luigi mused. "And old Elvin said that boos were behind his basement riot too..." He gasped. "You're trying to tell me that YOU'RE the masterminds? I knew it!! You guys are here to steal my supply of toilet paper!!"
"NO!! We're here to kidnap you!!" A pair of boos held up a giant burlap sack. "Get in the bag!!"
Luigi crossed his arms. "I don't wanna. And if you wanted to catch me so bad, why would you spend all night hiding out in my storage hole?"
Another weird pause. "Uhh... that was the king's plan!! Don't question his brilliance you nitwit! Now, in the sack!"
"And where IS Mario, huh? It would save me a lot of time and tears if you told me now."
"For the last time, YOU DONT GET TO KNOW!! Our suspicious hint is supposed to eat away at your interior regions!"
Luigi squinted. "Well that was metaphorically graphic."
"WHO GOES THERE??!" A huge deep voice boomed up from the storage hole, startling them all like New Donk pigeons. "Is it the dweeb?"
Booscaster jumped up. "Yes your majesty, it is the dweeb! He's finally here!"
"Well it's about time," the dark figure cackled, floating up out of the shadows.
And who do you think it was hiding out in there? What mysterious final boss might be waiting in the—
Okay we can stop pretending, its King Boo already, alright?! It's King Boo, he's the secret villain that everybody already knew anyway! Why are we even bothering to pretend we don't know still?
Man. I spend all this time trying to work up a sense of mystery over it just to have you people spoil it all before we're even halfway through. I hope you're happy.
With bejeweled crown of fab tilted on his head, looking like he'd just got off a Netflix binge, the slightly larger boo with a degree in epic anime emerged from the floor. "I've been looking forward to this... WE'RE GONNA KILL YOU—!"
Then King Boo caught sight of Luigi and the entire room froze. Their eyes met for a brief, brief millisecond, in which the entire planet stopped spinning.
In the background, a trio of boos suddenly started to sing that nemesis song from that Phineas and Ferb episode:
"Oh search your heart please Mister B,
and I am sure that you will see,
that you were always meant to be
my only lifelong enemyyyyy...!"
(See here.)

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

In that millisecond, they got in a few more good moments of gawky staring.
'Is that... the final boss??' Luigi started panicking. 'Oh no. I still have that stoopid glitter in my hair and now the final boss thinks I'm an idiot!' he thought. 'Stoopid Stoopid Stoopid!'
Meanwhile the king had started interior monologuing. 'I never would've expected. Is this shrimp seriously... wow. He looks like he jumped right out of the 1980s. And his overalls are too big for him. Idiot.'
Then the moment ended and the planet resumed spinning, sending the millisecond crashing into the next one.
Luigi blinked and gasped. "What just happened."
King Boo had already moved on and was staring blankly at something else. "...Kevin."
A boo with yellow eyes floated forward with a sigh. "Yes."
"What's that on his back?"
"Uhh..."
"NEVERMIND HE'S GOT A VACUUM!! It's E.GADD'S POLTERGUST 3000! EVERYBODY SCRAM!!!" King Boo hollered in a random outburst of uncharacteristic screaming. At his scream, every other boo in the room started screaming too, except their voices were a cacophony of higher-pitched shrieks like baby birds. Luigi screamed too for good measure.
"AAAAAAAAHHHHHHHHH!!!"
Like a conference room after someone let one rip, the room instantly began to empty as boos fled through walls and floors and ceilings. Their departure was accompanied by a blast of loud obnoxious techno music, which faded as soon as the last boo was gone.
And just like that, the whole boo cohort cleared out.
And then Luigi slumped back against the wall, breathing like a walrus.
"Well, that happened," Mappy remarked.
"Did it? I think I passed out," Luigi said.
"Oh, come on! Whoooo let the boos out?" LR suddenly belted out from the mirror. "WHO? WHO? WHO-WHO?? WHO—

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

"Oh man, he even sings like you," Mappy groaned.
"Alright alright! You told me to open it!" Luigi pointed at the mirror. "And what was going on there? I feel all weird and important inside... like for just a minute, my aimless life finally found real purpose!!"
Mappy snickered, but LR stepped forward supportively. "Yes, I saw it too! Don't you know what this means?? Luigi, you have finally found your arch nemesis!"
Luigi gasped. "My what?? You mean I get one too?? Oh man, wait till I tell Mario! He'll never believe me!" He pumped his arms. "I knew it!! This will be a day—
MEEMAWMEEMAWMEEMAWMEEMAW—!!
"Aahhh!!" Luigi screamed again as the Gameboy Horror started beeping in his pocket. "Oh no! The old dude is calling! Answer it!" he said.
"Dummy, I'm an intangible reflection," LR replied.
Grudgingly, Luigi picked up the machine and punched the start button. "Yeeeello. Hey Gadd, guess what?!"
The professor ignored him. "Loogy! That was them! Those were the fiends who stole my portraits and wrecked my lab and ruined my dorm party!"
"So it doesn't matter what I say, I see," Luigi grouched.
"I wondered where King Boo and his gang had gone," E.Gadd went on, drinking a bottle of Coke.
Luigi narrowed his eyes. "I can see you're really concerned here."
"Oh I am! Thanks for flushing 'em out, Looshy! But..." he coughed. "Perhaps it would've been better if you hadn't let every single one escape. Just a thought."
"Look, I had no idea!! The man in the mirror who looks like me told me to do it, I swear!" Luigi argued.
E. Gadd blinked, which we couldn't see behind the glasses. "Yeah... well, water under the bridge. Why don't you come on back to the lab for a sec?"
"Why, so you can look at my head?" Luigi asked.
"Ye— uh, no. I just need you to come back already!! Stop questioning your elders!! Hurry up!"
Jolting up in some kind of terror, Luigi juggled the Gameboy and ran for the door. "I'm coming I'm coming!!"
Behind him, LR crossed his arms. "Well that was anticlimactic. See you later, boo-boy ...WHO! WHO! WHO-WHO—
We're changing the scene now
When Luigi passed through the foyer on his way out, Toad jumped up from his seat by the doors. "Luigi! There was this big boo flying around in here! Be extra careful!!"
Luigi didn't even stop walking. "Well why'd you let it escape?!"
Toad blinked; this had never occurred to him.

*FLASHBACK*

Toad was smashing buttons in a violent Pokémon something or other when all at once a boo busted through the walls, a wild and crazed look in his eye. He sat there for a second, glanced around, then caught sight of Toad sitting in the corner.
Toad promptly pulled a fifty dollar bill from his wallet and held it up. "I'll give you this to turn the other way."
"Done." It snatched the money and left.

*a short time later*

In a secret unidentified room, King Boo was evilly eyeing Mario, who was trapped in an unspecified way in the corner. "Well, our Netflix binge is over. Guess it's time for the good part, fleshface," he announced.
Mario shifted. "Oh goodie. Are you ready to pass to the next world already? Pretty sure you don't belong in this world anymore."
"Strong words for a human who is clearly out of his element in the middle of BOO WOODS," the king said pleasantly. "Anyway. Kevin, bring the you-know-what. We have to do that thing we bluffed about."
Boolossus sighed in the corner.
"Mario, hold still or you might lose a limb," King Boo instructed.
"Eat me," Mario replied.
Just then Boonswoggle busted into the room. He waved a paper above his head with a stubby hand. "I just made fifty bucks!!"

*END OF FLASHBACK*

Toad grinned. "Don't worry, I'm sure you'll get him, Luigi!"
Luigi snorted and walked out the door.
When he came back into the lab he found E.Gadd sitting by his computer desk playing an online pinball game.
"Louie! Finally! I've been waiting for you!"
Luigi frowned and slouched over. "What do you want? Are we going to empty the vacuum again?"
"No. I saw what happened in the storage room."
Luigi was disturbed. "Yeah, about that! How can you see everything I'm doing? We're running off 2001 technology!"
"That's for me to know and you to never find out. So, your brother must have been taken by that gang of boos... poor feller."
Luigi crossed his arms. "Wow, I didn't know you actually had a sympathetic side. And by the way, those boos? One of them is my new evil nemesis! The king one!"
"Don't be ridiculous, Lyle. It takes years of animosity to get a nemesis. ESPECIALLY King Boo. He's off limits anyway."
"Why?"
"Because he's MY nemesis!" E. Gadd snapped. "We've been at each other's throats for decades, boy. What makes you think you can just slip in here and steal him?"
"But the boos and my reflection said so!" Luigi whined.
E.Gadd shrugged, clicking away at his pinball.
Luigi glared. "Hey, listen up, friend. He's mine! Got that??"
"If that satisfies your little imaginative brain," the professor replied, never looking away from his screen.

E.Gadd: Why does this matter so much to you anyway??

Luigi: Because... because!

LR: He's insecure. Don't know what for. He's turning heads when he walks in the door.

Mappy: Ain't that the truth.

Luigi: WOULD YOU JUST... STOP SINGING AT INAPPROPRIATE TIMES??

Mappy: Haha. He's only your own reflection. Take a look in the mirror, Sherlock.

Luigi: Nuh-uh! I would never sing about my emotions like that!

Mappy: Sure you wouldn't.

Luigi: Besides, I am absolutely not insecure. I just think it's cool that I have a—

King Boo: A what?

Luigi: Uhm, uh, uh, uh... KING BOO! TELL THEM YOU'RE MY NEMESIS!

King Boo: Well it's a bit early to be making calls like that, isn't it? I mean, we're only in chapter 14. And the Internet hates spoilers.

Luigi: Grrrr!!

Luigi fumed and almost started another argument, but got ahold of himself. "Well anyway, they're probably gonna eat Mario or something!"
E.Gadd didn't look up. "Oh now, don't talk crazy. They're not gonna eat the man. ...They never said that. Anyway, what did become of your brother, hm? Odd that he hasn't come back."
"Well if they took him, it's not really a big surprise, is it?" Luigi asked in irritation.
E.Gadd ignored him. "Common theory holds that when boos gang up, their power grows. Their power likely overwhelmed your brother."
"Overwhelmed HIM? What about me??" Luigi asked.
"Hehe! Don't get all down in the mouth, my boy! That GBH I gave you has a secret function! I knew it would come in handy someday, so I developed it secretly. It's called the Boo Radar!"
"Why did you develop it secretly?" Luigi asked.
"Because Thomas Edison and I were in an invention war at the time and I didn't want him to steal it," E. Gadd waved him off. "Anyway, this thing will beep and flash red whenever a round object like a boo is nearby. Use it to search everywhere!"
"Everywhere?"
"Everywhere," the professor repeated. "Now don't try to stress yourself out. There's only like thirty-something boos in the mansion anyway. Just calm down and focus on getting them one at a time."
Luigi stood and stared at him doubtfully for a moment. "You're being awfully nice all the sudden."
"NOW GET BACK TO THE MANSION! GO GET MY PORTRAIT GHOSTS!! HURRY UP YOU KID, TIME IS WASTING!!" The professor kicked him towards the door.
Luigi jolted. "What? You mean that's all you wanted?? To talk to me for two minutes?!"
"Yes, got a problem?" The professor asked.
Luigi fumed for a minute, then grabbed the doorknob. "Well... a Bah Humbug to you too!" he said indignantly, slamming the door.
Behind him, E. Gadd snorted and sipped his Coke. "Kids."