"If you want to go on an underwater journey, Gillyweed is undoubtedly your best option.
It lets you swim freely in deep waters—unlike the Bubble-Head Charm, which can burst under water pressure if you dive too deep.
Nor is it like some clumsy Transfiguration, which makes swimming unbearably painful.
Of course, if your Animagus happens to be a fish, you don't need to worry about any of this!"
—Eladora Ketteridge, Magical Water Plants of the Mediterranean and Their Properties
...
Jon could personally attest to the joy Gillweed brought.
Especially since he had an Ashwinder egg to warm the water and a Patronus to lead the way.
Naturally, he didn't forget to recast a Freezing Charm on the Ashwinder egg every few minutes; otherwise, if it ignited fully, he'd be the one getting scorched.
He reminded himself that he was here for the Second Task—not a pleasure dive.
...
Jon swam through the pitch-black lake, his path lit only by the faint glow of his Patronus. Silence pressed all around, broken only by the flick of tiny fish darting past.
The Patronus made no sound, floating dutifully in its bubble as it scanned the waters.
"Woo!!"
Suddenly, the Patronus snapped its gaze toward Jon and huffed a sharp warning.
Jon understood instantly. He dove down, tugging the bubble with him.
Moments later, a massive shadow glided slowly overhead.
The sheer size of it made Jon's heart lurch—it was nearly as big as a Ukrainian Ironbelly!
A giant squid. The apex predator of Black Lake. In its domain, it was perhaps even more terrifying than a dragon.
Jon pressed himself into the lakebed weeds, barely daring to breathe.
Fortunately, giant squids weren't adept at deep dives, and their eyesight wasn't great. Within minutes, the beast swam off.
If not, Jon might have met his end right there.
...
He swam onward.
Ahead stretched a patch of emerald-green water plants, nearly two feet thick, resembling a dense underwater lawn.
"Woo!!"
The Patronus huffed again, eyes fixed warily on the swaying plants.
Jon nodded, raised his wand, and with a flick—bubbles streaming from his mouth—whispered:
"Diffindo!"
Several stalks sliced clean apart, revealing grotesque faces hidden within.
Pale-green creatures with horns, like aquatic demons...
Grindylows.
These water-dwellers of Britain and Ireland fed on small fish, but often attacked both Muggles and wizards. Their long fingers were their main weapon—
—and their weakness. Professor Remus Lupin had explained it in second-year Defense Against the Dark Arts: "Though powerful at gripping, their fingers snap easily."
Several Grindylows lunged toward Jon.
"Relashio!" Jon shouted calmly, a stream of bubbles escaping his lips.
A jet of boiling water blasted into one, its pale-green claws turning red.
"Relashio!"
"Relashio!"
He drove back three Grindylows in succession before the Patronus fell silent again.
Jon exhaled in relief and pressed on, past the weed bed.
Darkness closed in fully now, silence weighing on his eardrums. He had reached the true bottom of the lake.
The Merpeople village couldn't be far.
...
The Patronus suddenly darted off in one direction.
Jon followed close behind—his Patronus had found it.
They swam swiftly. Time slipped away quickly too. Jon checked his watch—fifteen minutes had already passed.
Then he heard it—
*"We've taken your most precious treasure.
You have but one hour..."*
The Merpeople's song echoed clearly now, and Jon felt his pulse quicken.
"After an hour, all hope is lost..."
Enduring the harsh melody, he soon spotted rough stone houses below, draped in algae.
Merpeople stood outside the dwellings—creatures with human torsos and fish tails.
Their iron-gray skin was mottled and harsh, their long green hair tangled like weeds.
Yellow eyes gleamed above broken yellow teeth, and crude necklaces of pebbles hung around their necks.
Ugly—male or female, every one of them was hideous. Fairy tales had deceived them all.
...
More and more houses appeared. This was clearly the Merpeople village.
Crowds of them swam out, staring curiously at Jon, pointing at his webbed hands, his gills.
Like city folk mocking a poor villager.
Soon, Jon reached the village center.
A massive statue loomed ahead, carved from a single stone. Crude, but unmistakably a merperson.
Four figures were bound tightly to the statue's tail.
All four seemed deeply asleep, heads lolling, fine streams of bubbles drifting from their mouths.
Astoria lay across Hermione's lap, with Cho Chang at her feet. On the far side, a silver-haired girl of eight or nine—Gabrielle, Fleur's little sister.
Jon swam toward them with all his strength. The surrounding Merpeople did not stop him.
The trial seemed to end here.
By the captives stood a merperson taller than the rest, wearing a crown of shells and a short necklace of shark teeth. The leader.
"You may take only one..." croaked the Merchieftainess, gesturing as she spoke. "...You were the first to arrive."
Jon's little Panda edged closer. The Merpeople didn't stop it—some of the females even cast it tender, maternal looks.
"Sorry... until now, I didn't have a choice," Jon said with a shrug, raising his wand.
"But now—I'm taking them all!"