It was a secret passed down from the very Founders of the school, written down by Salazar's shaking hands as the prophecy left Rowena's mouth, passed down from the mouths of ghosts who were relieved to have crossed on before that fateful day. The day that doom itself would come to Hogwarts School of Witchcraft and Wizardry:
"Hey, Gred, look at that game all the Muggles are playing."
"Are those some kind of magical animals on their fellytones?"
"Looks like it."
"And they just wander around? Looking for them?"
"It would appear so, oh brother of mine."
"Dangerous, that, Forge."
"Dangerous indeed."
"How fast d'you think we could get that into school?"
/
No one knew how it truly happened. The legendary Marauders, back from the grave for one last, ultimate prank, perhaps. A Muggle Studies assignment gone horribly awry. All they could say, in the After Days, was that no one had realized how bad it would be.
That wasn't quite true of course. One person had.
"THE PROPHECY IS NIGH! IT IS NIGH!"
McGonagall jerked and almost stabbed Professor Flitwick with her butter knife.
"Oh, Sybil, it's too early for this!"
Trelawney paid her as much mind as usual, which was to say none at all, and proceeded to continue shrieking about the End Times as she stared across the Great Hall in horror. McGonagall rolled her eyes (she couldn't reasonably be expected to deal with this until after her morning tea, she just couldn't) and tried to continue eating. Really, it didn't look much out of the ordinary from a regular day. Some students were bent over those glowing muggle contraptions, but nothing bad. Certainly nothing dangerous.
/
The first casualty took place before the day was out; one very invested Ravenclaw walking straight off the Astronomy tower.
Within a week the school had fallen to chaos. Before long the Ravenclaw was not the only one in the infirmary; no fewer than ten students had been carted in after falling from their brooms while attempting to chase down rare Pokémon. This was fine with them, as the steady stream of students kept the lure at the Pokéstop from running out. They would catch the shit out of that Chansey.
The ghosts stopped being smug about not having to deal with this shit approximately five seconds after someone (Peeves. It was Peeves) started the rumor that they spawned ghost-type Pokémon. Moaning Myrtle flooded the girl's lavatory, the Grey Lady hid in the tower, all to no avail.
The same someone (Definitely Peeves) was probably behind the rumor that Poison Pokémon would spawn whenever someone botched a potion.
The centaurs were in a rage as foal after human foal tramped across their Forest, in search of the elusive Scyther just three footsteps away. The humans would soon learn the folly of their ways, however; just as soon as they were done amassing their army of Ponyta and Rapidash. Oh, how the men would pay.
Some asshole placed a lure on the Whomping Willow and wouldn't let it run out. Real bright idea. Jerk.
Poor Mathilda Thinwhistle splinched herself during the Apparation lesson when she tried to be the first to have a Farfetch'd by making it all the way to Japan.
"Gotta catch'em all!" was all she could say as Madame Pomfrey tried to grow back her phone-holding hand. "Oh look- The Chansey, there it is!"
Grass types in the Greenhouses. Pidgeys in the Owlery- well, Pidgeys everywhere, actually. There was no getting rid of them. Never had the Bubblehead charm been so quickly mastered as when a Tentacruel spawned at the bottom of the Lake, the students surging past a bewildered and bemused Giant Squid.
Hagrid was no help at all, not after one incredibly thoughtful first-year showed him how to acquire a Charizard, Dragonite, and Aerodactyl.
"You know that last one's a dinosaur, not a dragon, right?"
"A what- you muggles and your names for ancient dragon bones, bless!"
Proud pureblood Slytherins wrote home swearing to their parents that they hadn't fallen victim to this terrible Muggle malady. Well, they had to spell their quills to write it for them; their fingers were too busy throwing Pokéballs at that Ghastly in the window and not missing how dare you not register that toss, just you wait, Niantic, you wait until my father hears about this-
And then there were the traumatized first-years after someone (DAMMIT PEEVES, ENOUGH) started a rumor that Professor Willow was turning the discarded Pokémon into potions parts.
Professor Sprout thought they were harmless enough, a bit cute in fact. She kept a few Oddish and Tangela in her Pokédex, she would admit. McGonagall started summoning the fellytones at the start of class. The Muggle Studies professor was unbearably smug about the whole thing. It was annoying, but not more than Hogwarts could handle.
And then people were allowed to choose teams.
Full. On. War.
Pity the poor fool in Gryffindor who wore a blue scarf in support of Mystic, or the oddly foolish Ravenclaw who'd chosen Valor. One enterprising student temporarily stole the Sorting Hat to determine which team they belonged to, and had to spend first an hour explaining the game to the bonnet and then being sorted with far more care than the Hat had put into their living arrangements for the next seven years.
The Slytherins were too busy whining about how unfair it was that there wasn't a Team Green, why wasn't there a Team Ambition anyway, can't we all be Death Eaters- but it wasn't the Gryffs or the Claws that were the real problems anyway.
It was the Puffs that struck fear into the hearts of the others. Nothing combined into quite the same level of terror as the games fanaticism, competitiveness, and hard work. The upper levels began taking shifts to wander the castle at night, leveling up to dominate in the gyms.
One positive note, though, in terms of inter-house cooperation:
"You think you can beat me in a fight, you filthy Gryffindork?"
"You can't even aim your wand, you slimy sna- Wait a second. Did a Snorlax just spawn for you too?"
"Oh sh- truce?"
"Truce."
/
"-out of bounds again, O'Brian, I'm afraid that's twenty points from Ravenclaw this time! Now go to bed, and don't look at your phone until you get there!"
"Sorry, professor," muttered the boy, shoving his phone in his pocket. He had been so close to winning the gym, too!
The professor watched him go, face set sternly, only dropping the harsh expression when he had turned past the statue of Gregor the Grumpy.
The door opened in front of them just as they had pulled their own phone halfway from their robes. Professor McGonagall looked over them, watching as they shoved it back into hiding.
"Making the rounds, Pomona?"
"Too many students out of bed with this game still so popular," she explained.
"Too many," agreed the Head of Gryffindor.
"I have to make sure that my students aren't wandering around fighting gym duels? Battles? Whatever they're calling it these days."
McGonagall didn't move from the doorway. "Of course." She nodded once, firmly. "By all means, continue."
They stared at each other.
Then as one, they both whipped out their newfangled fellytones.
"This gym belongs to my Puffs! They worked hard for it!"
"Not my gym, you bitch!"