Disclaimer: I do not own Harry Potter or associated works, and any content recognizable or otherwise. This work is not for profit.
Harry and Dumbledore appeared on a worn street, and he could see the Gaunt house, more of a shack than a proper living space, in the distance.
"We shall approach carefully; Tom has no doubt put enchantments in place to bar those who wish his soul harm. Follow my lead, and touch nothing, especially if you feel oddly compelled to. You have experience fighting the Imperius and the mental control of Voldemort, and I am sure we shall be fine."
Harry fidgeted nervously. "Are you sure I'll be able to handle this?"
"Tom's greatest weakness is in his arrogance. He likely expects none to find out his secret, and even then, will likely have brute force style enchantments surrounding it, as he values power above all else. There may be some cunning to it, some dangerous traps we must be aware of, but he prefers not to utilize fully the intricacies of magic, which I know much of.
"I confess, I considered bringing Alastor along - he knows that of the Horcruxes - or even going alone, but decided that you should have a pivotal role in this coming fight, and what better way to prepare than to face a creation of Lord Voldemort himself, albeit watered down?"
"How will we destroy it?"
"Basilisk venom or cursed fire should suffice. The Sword of Godric Gryffindor is thus the safest option, except that it is much too heavy. Thus, I have brought along a protected vial of potent venom that I harvested when you killed it, as you may remember."
Harry did remember letting Dumbledore into the Chamber of Secrets to survey the damage. After finding nothing useful beyond the basilisk's meals, the man had decided to leave the Chamber closed. Venom was definitely better than the cursed Fiendfyre, which Moody had told horror stories about.
One concern remained. "But what if the vial breaks, like Slughorn's memories?"
"I believe Horace was using a defective vial to house his most precious memory, which was most unfortunate. I have taken the liberty to apply many of my own charms to this to make sure the poison does not leak, and I assure you that no harm will befall me because of it."
By then they had reached the shack. The wise old wizard raised his hand.
"Wait. I sense nothing, but it may be a trap." Muttering long incantations under his breath and waving his wand about, Dumbledore slowly stepped forward, pausing here and there and leaning his head as if searching for delicate heartbeats of magic in the ground and air.
He made his way slowly towards the shack, beckoning at Harry to carefully follow his footsteps, and then, suddenly, he blew the door down with a powerful curse. The precarious house creaked and groaned, its walls visibly leaning, but did nothing.
"There are no protections here," said Dumbledore, "but I sense no dark taint either. The Horcrux must be more carefully hidden, or I was mistaken about Tom's choice of hiding place."
He walked back and forth along the length of the dilapidated house with boarded up windows, tapping his wand on the walls, ceiling, and floor. Finally, he paused. "Aha! I have found an entrance of some sort - it is likely where the artifact is hidden."
He muttered a few more words, then frowned. Turning his wand upon his hand, he muttered, "Diffindo" and a single drop of blood fell to the floor on the spot he indicated. A groaning sound erupted from the ground, and Harry and Dumbledore jumped back as the ground within the shack opened up to reveal a large and deep pit with stairs around the edge.
"Underneath the floorboards, then. Follow me, and be careful where you place your feet."
After lighting their wand tips they together descended into the bowels of the earth, closer to whatever fiendish trap Voldemort had devised. The cold packed earth wall gave way to rough stone which jutted out from the side at times, forcing Harry and Dumbledore to duck or stretch their legs over the obstacle. The stairs themselves, made of wood, creaked at odd intervals and threatened to give way at times. They seemed ominously alive. At more than one point, Harry broke through the wood and nearly fell to a premature death. Their progress was hindered by Dumbledore's scrupulous checks for traps and triggers, including those which might collapse the structure further down or other such nefarious but less detectable possibilities.
But as Dumbledore had shared with him, it was likely that if Voldemort trapped someone, he'd want to be able to interrogate what was left of them to see how they learned of this place. One thing was sure, they would definitely find the Horcrux down here.
Finally, they reached the bottom of the seemingly endless staircase. The entrance was barely a distant ray of light. Dumbledore walked up to the wall.
"Another entryway," he remarked, "which exacts the same price. Tom never understood that there are far greater and far worse things than mortality and mere physical harm."
The man pressed his cut to the wall, and it registered the addition of blood. The door in front opened, but they could simultaneously hear an echo of the one up top grinding shut.
"How are we going to get out, Professor?" asked Harry.
"I do not know, Harry. I will admit that Voldemort managed to sneak that trap by in my complacency; I will do better to anticipate his moves in the future. Currently, though, we can do nothing but move on. I doubt the stairs will still hold. We shall find another way out of this cave."
With that, they continued deeper into the abyss, wands aglow.
After following a passageway for some time, they entered a large and wide cavern with smooth walls and seemingly no exit.
"The walls are too smooth to be natural," noted Harry. "Voldemort created this."
The sinister name echoed throughout the cavern. Voldemort, Voldemort, Voldemort, it whispered back.
"But where is the artifact, the Horcrux?" he continued. Horcrux, Horcrux, Horcrux.
A heavy stone dropped into the passageway behind, closing their only path of retreat, and a large door opened in the far wall.
"I have failed again, it seems, to detect Tom's traps. We cannot apparate away."
Out from the door lumbered a large, reptilian creature. It was much larger than the Hungarian Horntail he had faced during the First Task of the Triwizard Tournament, and looked much fiercer. The heads. Instead of having feet, each of its four legs split into around fifteen large snakes around half the size of the basilisk he faced in the Chamber of Secrets. The beast blinked, briefly obscuring its brilliant yellow eyes.
Harry became distinctly aware of Dumbledore shouting. "Look away, Harry! Its very gaze will slay you!"
"I'm not dead," he said, confused. "What is it?"
"It is one of the vilest creatures ever to exist in concept or in reality. The product of Herpo the Foul's terrible imagination, it trumps even the basilisk and cockatrice in terms of danger. I have only heard of them, for the magic required to actually create one is so dark that none, even Grindelwald and Emeric the Evil, were even tempted to try, for it requires the creator to make a Horcrux and partially integrate it into the creature's thought processes, allowing it sentience and magic...
"Herpo the Foul called it the Magnificent Beast. It will be the most intelligent hostile beast you or I have ever encountered or ever will. But I am puzzled as to why you have not died due to its sight."
"Maybe because I'm a Parselmouth? Or share a connection with Voldemort?"
"Perhaps..."
The beast finally fully entered the cavern, and the door behind it closed. It began to speak to Harry.
Who disturbs my slumber?
"I do," said Harry, nervous but trying not to show it.
Why are you here?
"Tell it that we seek a dark artifact. I doubt there is a way to avoid confrontation, so it is best to be to the point."
"You understand Parseltongue?"
Dumbledore merely smiled, a conjured blindfold covering his eyes as he held his wand aloft.
"I seek an object of the dark," responded Harry to the creature.
For what purpose do you seek it?
"I wish to destroy it."
You cannot do this, Speaker.
"Harry, now!" shouted Dumbledore, and they both fired blasting curses at the creature.
The snake feet rose up to protect the front legs, and opened their mouths. Two of the heads swallowed the curse, before emitting it back at them from different snake mouths as they hissed and squirmed.
"Look out!" yelled Harry, jumping to the side. Dumbledore, evidently having felt the shift in magic, jumped the other way.
"I think it can redirect our spells," shouted Harry.
"Perhaps an internal lining, charged with magic?" mused Dumbledore. "And I doubt it can be felled by a Killing Curse either, considering that it is not truly alive, but rather is a form of being that is supplemented by the existence of Voldemort's Horcrux. No, we must destroy it the conventional way, and seize the Horcrux."
Kill, tear, rip...
The beasts lunged forward, and the multiple heads on its feet reared back to strike. Harry shielded himself from the impact, and Dumbledore waved his wand, tying up some of the heads in a knot. Yet more shot forward, and he darted nimbly away from the monster, separated by the creature from Harry.
The beast turned but the onslaught of snake heads against Harry did not cease even though the beast focused its attention on Dumbledore in front of it; its back legs continued to tie Harry down underneath his shield.
Suddenly, the heads drew back. Harry scrambled to the side, dropping his shield, just as a rush of roaring flame consumed the spot he had been moments ago. Despite not being directly hit, he could feel a heat as scorching as dragon fire, and suddenly realized that he and Dumbledore were outmatched against this monstrosity.
If there was anything positive about this, it was that this required a Horcrux to operate and that Voldemort probably only created one. Probably.
The heads swiveled, and the roiling flames moved towards him. Dumbledore, on the other side and facing the biggest head of the creature, was probably having a harder time, if any. "Aguamenti!"
The pitiful gout of water quickly turned to steam when the scorching hot fire impacted it, evaporating Harry's defense. He needed better cover, like he had during the Triwizard Tournament, but there was no debris lying around...
"Bombarda!" The explosion fractured the rock on the wall to his side as he desperately ran from the hissing and jeering snake-feet abominations and their flames of death, but none of the pieces were large enough to hide behind.
"Bombarda!" More dust and fragments of rocks littered the cavern floor. He and Dumbledore were now nearly at antipodal points of the circular room, and the beast could only reach far enough to attack one of them at a time, thankfully. It chose Dumbledore. The old wizard in flourishes and waves summoned and transfigured many of the fragments Harry had made on the ground into sharp pieces of metal, and they flew towards the back of the creature, which continued to spew bursts of fire. Many of the pieces rebounded off of the thick skin of the creature, but some found their way into the opened maws of the snakes. Without even being able to see the metal shards he transfigured, the former Headmaster of Hogwarts and master of Transfiguration increased their sizes and all of the snake heads on Harry's side of the creature exploded in bursts of blood.
Harry, meanwhile, began to transfigure the larger pieces of rock he had blasted off of the cavern wall into durable cover, stretching and elongating the stones and molding them into a single piece between Harry and the Magnificent Beast.
He finished just as the decapitated heads grew back and began to spew flame in larger quantity. Dumbledore continued to frantically transfigure stone into weapons and stave off the blows and strikes of all the different heads.
"Harry, we need to destroy the beast from the inside!"
"I don't think anything could stand the heat at the very center of that thing!"
Hopeless, foolish, impossible...
"We must fight fire with fire, Harry! Cover yourself!"
Harry's eyes widened. Surely he couldn't mean...
He watched in awe as multifarious heads of flame blossomed out of the old wizard's wand. The animalistic Fiendfyre danced around the cavern, quickly spreading and consuming everything it touched with the exception of the Magnificent Beast, which continued to attack the former Headmaster despite the intense fury of the flames. But though the skin of the creature could keep the flames out, the raging spirits of fire sought out other entrances, forcing its way inside the mouths of the snakes and slowly but surely working its way up the monster.
Deprived of its favorite tools, the beast attempted to lunge forward, baring its teeth at Dumbledore and trying to crush the man with its weight, but with its feet mostly disabled it barely managed a minor flop. Dumbledore ran around it, still controlling the flames, to Harry.
You want to help me.
"I want to help you," murmured Harry, dazed and raising his wand to try to extinguish the flames. Next to him, Dumbledore did the same and the fire creeping up the beast eased as he reined it back. "I want to -" Wait.
He most definitely did not want to help Voldemort's Horcrux continue to live. "Professor!" he shouted. "Don't give in!"
The shout jolted Dumbledore out of it, but it was too late. With a roar, the cursed fire turned around in its path, raging towards Dumbledore and Harry. The Magnificent Beast had wrested control of the fire from Dumbledore during its last-ditch attempt at mental manipulation.
Dumbledore molded Harry's shield from earlier, extending it to form a barrier that would temporarily hold off the fire spirits. All of this he performed still with a blindfold.
"What do we do?" asked Harry.
"We must regain control," said Dumbledore. "But I am much too tired, mentally, to perform such a feat. You must do it, Harry, and confront Lord Voldemort's soul."
"How?" The meager shields of transfigured rock from debris were about to let the fire in.
"Look deep into the eyes of the flame, into the soul of the flame, seek out the rage which drives it, and discover its source."
"What do you mean -"
The fire ate away the last of the stone surrounding them, and the figures of flame reared back, poised to strike. Harry locked eyes with a flaming serpent, looking into it and farther, searching, deeper -
Destruction. An all-consuming fury, a burning passion -
And then Voldemort. Images from his past, from Tom Riddle's past, horrific, corrupt, vile ideas of all sorts twisted about in his head as the Horcrux battled him for control. It was winning, overwhelming him with hopelessness and despair.
Winning but for one thought: You have sealed your own death, Tom Marvolo Riddle, by committing such atrocities. I will live on in my friends and the Order, in all that is good in the world, but you - complete and utter annihilation is all that awaits you. I am not the hopeless and despairing. You are. If I die, then when the eternity of existence finally consumes you, I will be there. Waiting. Triumphant. If you flee from Death, he will only flock to you all the quicker.
And then exhaustion. The flames returned to their original course, screaming up the veins and passageways of the Magnificent Beast, charring it from the inside as Harry and Dumbledore, who finally ripped off the blindfold, looked on, until finally the whole thing let out a shuddering sigh and slumped down. Harry, with the help of Dumbledore, eased down the flames until they vanished.
Then the Magnificent Beast exploded, and out came a glimmering gold object - Harry caught it by instinct, curling his hand around the Horcrux in victory.
Then he felt immense, unbearable pain and black spots danced across his eyes. He fell to the floor, gasping, and watched his hand begin to blacken.
A/N: I know you guys might be wondering why Dumbledore doesn't use phoenix travel. I decided that it's too OP. Accordingly, I did away with it - in CoS Fawkes entered the still-open Chamber of Secrets entrance and in OoTP Dumbledore apparated out. Ultimately, plot drives the world here, not the other way around. Also, if you liked this scene in particular, I recommend my other two stories!