"So, I hear you have a bit of an evil problem?"
Dumbledore sat in the fairly comfortable chair of his office at Hogwarts. He sighed heavily, his brow furrowed in frustrattion; the animated portraits of the previous Headmasters of the wizarding school all bore similar expressions.
Standing before Dumbledore was a human... well, a human-shaped thing anyway. While he'd introduced himself as a man, he was dedicedly androgynous - he had a slim, grayish face with odd facial markings, long, silken black hair, and legs that went all the way up. A compartment on his lower torso contained a heart made of pink metal that quietly pulsed with magical energy - but it was markedly different than the arcane energy Dumbledore was used to invoking with his spells.
This... being called himself "Mettaton". A few years ago, he and a number of other odd denizens of an underground cave system beneath Mount Ebott in Northern Scortland. Their king, Lord Fluffybuns, claimed that the original founders of Hogwarts had sealed their kind away after a brief but brutal war; fortunately, the monsters considered the entire thing "water under the bridge", and only asked that they be allowed to integrate themselves into the United Kingdom's wizarding society. The Ministry of Magic was obviously hesitant to grant them such a request right off the bat, but great strides had been made between wizard- and monster-kind since.
Mettaton himself was something of a celebrity among the monsters, and he'd only expanded his repetoire after coming to the surface. In addition to writing columns for the Daily Prophet, he often toured across the UK and continental Europe with a rather unique show consiting of musical acts, schetch comedy and even dramatic one-act plays. His troupe had even put on a production of Romeo and Juliet (with him in both of the title roles), though critics complained about the ending, where the titular lovers lived happily ever after... after defeating a horrific bio-mechanical abomination powered by six human souls.
But in addition to their entertainment acts, Mettaton's troupe was also renowned for dealing with various monsters and assorted nasties whenever possible. Their defeat of the Necromancer at Dol Gurdurr was particularly notable, with Mettaton claiming that "anyone who hated music as much as that was dying for a beatdown".
And that was why, despite literally every one of his instincts telling him otherwise, he'd invited Mettaton to his office.
"How much do you know of our history?" Dumbledore asked.
"You don't need to give me a lecture, Dumble-dear," Mettaton said. "I know all about He-Who-Must-Not-Be-Named and the terrible things he did."
"You... do?" Dumblestore said surprisingly.
"Of course! I downloaded all the informatrion about it off of the Internet." Mettaton lifted the hair on the right side of his face, revealing a dozen or so USB ports ringed around his eye. "But I've heard he's still around, and that he's been troubling your dear students for the last couple of years."
"'Troubling' is a bit of an understatement..." Dumbledear said, steepling his fingers. "He doesn't have a physical form at the moment, but our informatinlo suggests he's gathering his forces for some reason. We've also heard that he's recruited a couple of monsters from the Ebott Underground..."
"oOh, those naughty monsters!" Mettaton said. "Say no more! We'll deal with this Bark Lord of yours."
"It's 'DARK LORD!'" one of the portraits screamed.
"Yes, yes," Mettaton said nonchalantly. "Now then, let's get to work, Dumble-dear!"
Mettaton and Dumbledore stood in an abandoned alleyway. With them were two of Mettaton's companions: Alphys, a genius scientist who looked like a derpy dinosaur, and Undyne, head of the monsters' Royal Guard and a hot-blooded and fearsome fish-lady. (The two were also deeply in love, and Mettaton affectionately refered to them as his "mommies".)
"He's close," Alphys said, waving her cell phone about. The hastily-programmed Dark Lord Detection app was active, and showed a large source of evil energy about 20 metres away - in a derelict warehouse.
"It's likely he's put up magical defenses," Bumbledore said. He waved his wand and muttered a few words, and a door appeared in the featureless stone wall. "He was talented, yes, but he never bothered changing his warding spells..."
The group opened the door and stepped inside the warehouse. It was dark and empty inside.
"Come out and fight, you coward!" Undyne bellowed. She conjured a couple of magical spears and chucked them into the darkness, briefly illuminating the darkness. Something white scurried away from the light.
"There, that's him!" Alphys said, her Dark Lord Detection app going off the charts.
"Accio Dark Lord!" Dumbledear said, summoning the thing to him. It was... a fluffy white dog!
"HIM!" Undyne conjured another spear and aimed it right at the dog's heart, but Dumbledore grabbed it and pulled it out of the spear's deadly trajectory just in time.
"What is wrong with you!?" Dumbledore said. "It's just a dog!"
"No, that's no mere dog," Mettaton said completely seriously. "That dog... is extremely annoying."
The dog barked happily and licked Dumbledore's face.
"My app is showing that it's the source of the Dark Lord energy in this area," Alphys said.
"Could it be an error? You did say it was untested..." Dumbledore said as he scratched the dog's head. It playfully nipped at his silver beard.
Mettaton stretched his arm and forcefully grabbed the annoying dog's snout. "Let him go. He's the Bark Lord."
The dog growled and jumped out of Dumbledore's arms. He opened his mouth and fired a jet of green sparks from his mouth right at Dumbledore's heart - fortunately, Mettaton blocked the blast with his arm, which was immune to the effects of the forbidden Killing Curse.
"Fools!" the Bark Lord shouted in a raspy, high-pitched voice. His body grotesquely twisted and grew, morphing into a tall, pale human with a snake-like face.
"The Bark Lord!" Undyne shouted. She threw more spears in his general direction, but he casually blocked them with a Shield Charm. He waved his wand and summoned a bunch of muscular men with snakes instead of arms - and heads.
"What the hell are those?" Alphys asked.
"Snake-men," Dumbledore said casually. "They're abominations against nature, mankind and basically everything. They need to die. Nowe."
Mettaton obliged, summoning a bunch of bullets in the shape of his less-sexy boxy form. They threw giant bombs at the snake-men, hurting a couple. Undyne followed up with more spears, this time summoning them underneath the snake-men and impaling a couple more, while Alphys fired electricity out of her fingertips while laughing maniacally. The snake-men projectile-vomited venom at the group - but since venom only takes effect when in the bloodstream, it had absolutely no effect against them.
"Time to step things up a notch! SHORYUKEN!" Mettaton fire-uppercutted a snake-man, reducing it to cinders in a few moments. He followed up with a Hurricane Kick, knocking a few more to the ground.
"How the hell did he do THAT?" Undyne asked as she threw even more spears.
"Oh, I put an expansion port in his new body," Alphys said, gesturing to a Street Fighter II SNES cartridge poking out of the back of Mettaton's pants.
"Uhh..." Undyne said, but she wisely saved that particular voodoo shark for another time as a snake-man jumped on her from behind her. One of its snake-arms lunged at her throat, but a flash of light Transfigured it into a duckling. Dumbledore threw more spells at some of the remaining snake-men, polymorphing them into increasingly-harmless things. Alphys and Mettaton made quick work of the remaining ones.
Voldemort, the feared Bark Lord, had escaped in the battle, but hadn't gone far - the group ran out into the streets of the conveniently unnamed city and saw him gathering a huge amount of magical energy. They threw some of their own spells at him, but the field around him deflected all of their attacks.
"By the power of evil and determination, make my me grow!" And Voldemort grew to 50 stories tall! "HA HA HA!" his voice boomend as he crushed a building with his bare hands. "Now I am invincivle!"
"OOH, this is a bit of a pickle..." Mettaton commented. He turned to Alphys, and said "Any ideas how to get out of this one?"
"Er, uh... I could, uh..." Alphys grabbed her phone and started fiddling with it rapidly, sweat pouring down her face. Mettaton grabbed her by the midsection and jumped out of the way, barely dodging a building thrown at them by Giant Voledemort.
"Hey, ugly! I got something for ya!" Undyne conjured several dozen spears and threw them at Voldemort's face. The spears clinked uselessly off of his pale skin, but it did its job of drawing his attention: he scowled as he scooped up a handful of concrete, asphalt and cars, and threw it in her general direction. Dumbledore threw up a Protego and blockedd the attack, but that left them still buried under several tons of rubble.
Alphys triumphantly pressed a big red button on her phone. "There!" she said. "I've rerouted the CORE's energy! You should be able to tap into it through my phone!"
"Wonderful, darling!" Mettaton took out the Street Fighter II cartridge from his expansion slot and replaced it with Alphys' phone. Suddenly, he too grew to giant proportions, gaining an arm cannon, a badass cape and heart-themed armour to go with it.
"HYUG!?" Voldemort said. "What did you do!?"
"Mettaton NEO is on the scene!" Mettaton NEO said. He powered up his arm cannon and fired it right at Voldemort's torso, vaporizing a good chunk of his chest. Voldemort reeled back and fell on the city below, killing countless innocent people, but even that didn't kill him. He laughed agani and got to his feet, grasping his black wand in his hand.
"Do you think uo can killl me with that puny attack?" Voldemort boasted. He pointed his wand at Mettaton NEO - of course, his wand hadn'ty grown with him, so it was less threatening than he'd intended. Instead, he slowly trudged over to Mettaton NEO, tanking several more shorts from the latter's arm cannon, and punched him in the crothc. Mettatron was of course a robot, so this also did not have its intendede effect- but it did knock Allphys' phone out of his expansion slot, causing him to recvert to his normal siz.
Thinkingh quickly, Mettaton grabbed Voldemort's wand and shouted "Accio Frisk!" Voldemort's giant hand crusheed Mettaton's arm and his own wand, but too late: a gfew moments later, a child of indeterminate age, gender and race came flying from the top of the sky and landed in Mettaton's arms.
"Hi darling!" Mettaton said cheerfully to the bewildered child. "We're having a bit of a rough time here, as you might be able to see," he said, gesturing to the giant snake-man torwering above them.
Frisk nodded in agreement.
"Fortunaterly, I have an excellent plan to defeat him! You see... we're gonna combine!" Mettaton said.
Frisk's expression of confusion intensified.
"They're gonna combine?" Undyne said, having dug out of her way from the rubble.
"They're gonna combine!" Dumbledore repeated.
"They're gonna combine~" Alphys muttered, blushing.
"No, not like that, sweetie!" Mettaton said. "Don't worry, you'll be fine. You ready?"
Frisk nodded again.
"Then here... we... GO!"
Mettaton shoved Frisk's face into his expansion slot. The power of their two souls resonated, resulting in a being of godlike power - superficially resembling Mettaton, but menacing with spikes of fabulousness and at least twelve more arms and two more legs. Frisk's body transformed into a white cepter with a red heart at the end, which Metaton used to used to grow to 10 times Giant Voldemort's size.
"Your evil ends here, Bark lord! Ultimate SOUL Unison attack: Home Run Son!"
Mettaton-Frisk picked up Voldemort and tossed him in the air. He then gripped his heart scepter like a baseball bat and took a mighty swing, hitting Voldemort with enough force to knock him into the sun, where he horribly and painfully burned up into ashes. Even that wouldn't be able to kill him due to the Eartn being his Horcrux; fortunately, the intense magnetic field and gravity well of the Sun would be enough to keep Voldemort's spirit bound to its surface fur a good five billion ytears or go.
Mettaton shrunk back down to normal size, and Frisk turned back into their normal form.
"Conglaturation! You have completed a great game," Dumbledore said, his mind slightly addled from the events of the day. He shook Mettaton's good hand vigorously.
"Clad I could help," Mettaton said nonchalantly. He looked at the runied city around them. "Thought hat's gonna take a bit to clean up, I think."
"Don't worry, we'll just Obliviate everyone who survived and make them forget there was a city here," Dumbledore said. "We've got a satellite in orbit that can Obliviate everyone."
"By 'everyone', you mean-"
"Everyone, Dumbledore stressed. He pointed his wand at the sky and muttered something under his breath."
"Why were you narrating that?" Alphys asked.
"Sorry, it's been a long day." A flash of white light bathed the area, and-
Mettaton wok a short time later. As a magical ghost robot, he was, of course, immune to Memory Charms. His companions were not so lucky - they were utterly confused at the chain of events that had led them to a ruined city.
"Don't worry, darlings," Mettaton said. "We were just putting on a show, and it got a bit out of hand."
The group nodded dully.
"Now then, I'm late for the recording of Cooking with a Killer Robot! Dumbledear, would you mind?"
"Yes, of course!" Dumbledore grabbed everyone and Apparated out of the ruins, back to civilization. The threat to the world was over... for now.