The Chamber's entryway was a long, dank tunnel, lit fitfully with an eldritch green luminescence. The walls were lined with carvings of snakes and serpents similar to the ones upon the wall outside. To the feline couple, it might as well have been full noon, as their cat-like eyes easily adjusted. Slipping off their footwear they moved cautiously and silently on.
The Chamber itself was a massive vaulted room, a dome in layout, although the ceiling was lost in the darkness high above. About the circular wall were several statues in which snakes featured prominently. Almost a third of the floorspace was taken up by a shallow pool, no more than waist-deep on a grown man, and across the pool was the largest statue, a bust of Salazar Slytherin himself, most likely. No other carving in the entire Chamber was of this scale, nor of this prominence.
Hermione was the first to spot Ginny, lying unconscious by the pool, with Riddle's diary in her outstretched hand. The posture of her fallen body suggested an act of last-minute rebellion, as if she was trying to rid herself of the spiritual incursion by discarding the diary. She'd managed that once already, but it looked like her second attempt was a failure.
Harry was more interested in the figure in what to anyone else may have been called shadows. As Hermione moved from the entryway to check on the fallen girl, the tall figure, a young man with ridiculously styled blonde hair, slid forwards, intent on picking up the catgirl's wand. Despite the shoes he wore, the boy made no sound. He didn't have a scent, either. The catboy's best guess was that this was Riddle, and letting the diary's projection get 'his' hands on a wand was probably a bad idea.
He very nearly flew from the shadows as he pounced, sailing across the intervening distance with ease, twisting as he did, both for extra distance and power, and to ensure his feet were under him when he landed. Slamming into Riddle's projection he shoved it back, stumbling, until it fell into the pool. At the noise, Hermione grabbed up her wand again.
We have to hurry, she yowled to her mate. She's alive, but she's fading fast. If this thing is possessing her, it's draining her life force and shredding her soul to make room for its own.
Harry's eyes narrowed. He didn't know this dying girl, except as his friend's sister. He figured she had a crush on him, her behaviour and the thing with the butter dish at the Burrow last summer made that obvious, but she wasn't his Her-my-honey, his mate. She was an innocent, tormented by this man, this thing that was too stupid to know when he should just get over himself and die. Harry had a saving-people thing, and for his friend's sister, it was in high gear.
Hermione liked helping others. She hadn't always been appreciated for it, but she always tried. It was a lesser version of Harry's instinct, perhaps, but her life hadn't been spent with people whose attitude bordered on outright abusive, so the urge in her had not been quite so well-honed. Yet it was there. The girl before her needed help, so she would help.
Riddle's projection finally got its feet under it, and rose from the water spluttering and incensed. "How dare he?" Rage-filled thoughts occupied its mind, almost overwhelming its sense of superiority. "How dare this half-human thing assault me? Harry Potter must die, as will any other such creature!"
As the projection forced its way through the water, dragging more memories from the diary, and more magic and life force from the girl, it roared at Harry. "Tell me, Potter, too scared of dying at the hands of the greatest wizard in the world to stay human? By all accounts you'd only stand out more like that. And oh, look, you've dragged a girl into it as your plaything, how Slytherin of you. Now if only you were the Heir of Slytherin, you might actually stand a chance of surviving."
"If you were the greatest wizard in the world, I might be," Harry retorted. "Of course you ran into the scariest, most dangerous creature in existence: A fifteen-month-old with a full nappy. Give it up, if anyone in this place is the greatest wizard, its Albus Dumbledore, you didn't even try to face him. No, you chose to instead face the big, bad, baby!" Riddle's projection lost it.
Dragging itself from the pool, the projected spirit was at least solid enough to be drenched by its little sojourn in the water. If the thing was solid enough for that...
Harry leaped forward, slashing with the claws on his left hand, as the projection threw itself back... into the water again. The green-eyed boy took the opportunity to draw his wand as he moved, so as Riddle's spirit surfaced once more, he aimed his wand and pronounced the spell for the Full-body Bind. "Petrificus totalus."
The projection wasn't affected, but Ginny was. 'Riddle' had a nasty smirk on its face. "Oh, yes, I suppose I should teach you something before I kill you..."
Hermione cut the monologue off. "Don't bother ranting like a Bond villain, Riddle," she yelled, "we're not even in an inescapable deathtrap. So whatever spells we hurl at you hit Ginny, huh? Obviously, physical assault might work, or you wouldn't have dodged Harry's claws. The only problem..."
Harry took over the train of reasoning. "...by the time you're solid enough to kill, Ginny would be dead. However, there's two of us. I can keep you busy, while my girlfriend works on destroying your diary. She's really good with fire magic."
The very thought of the diary's destruction brought terror to the face of Riddle's projection. It turned to the bust and hissed loudly: {Speak to me, Slytherin, Greatest of the Hogwarts Four!}
As he finished, the mouth of the statue lowered with the grinding of stone on stone, causing both cat children's ears to flatten back. Realising exactly what was inside the rapidly opening portal, both of the felines closed their eyes tight. They were hideously outmatched, well out of their weight class, but if they ran, Ginny would die.
{Slay them both, my pet!} 'Riddle' commanded. {They must both perish!}
The gigantic serpent obeyed, lashing out at Harry, who barely dodged by throwing himself towards the basilisk's blind side. As his whiskers twitched, he heard wingbeats, and the Chamber grew warmer, and more brightly lit, as Dumbledore's phoenix companion, Fawkes appeared in a burst of blinding flames. Blinding, that is, for 'Riddle' and the basilisk.
Hermione's eyes were shut, but her whiskers told her where the great serpent was, and as Fawkes dropped a bedraggled bundle of cloth and leather to the floor, she moved. Magic was pretty much useless on a basilisk of this size, so she extended the claws in her right hand as she leaped forwards, planting a foot on the edge of its mouth and launching herself towards the snake's face. Biting back the cry of pain as she scraped her foot against one of the creature's fangs, she drove her arm forward blindly, covering herself with blood as her claws ruptured the serpent's remaining eye. Then she realised what had happened as her foot exploded with a slow, burning agony from the snake's venom.
"You hurt it, it kills you, a very fair trade," 'Riddle" gloated. "The venom is quite potent, and there is no cure."
Harry's voice was flat and cold. "Riddle," he said, "shut the hell up, you're monologue-ing. If I wanted your Fiendish Plan to Rule the World, I'd find a way to beat it out of you." He caught a glimpse of silver as he opened his eyes, knowing his mate had blinded the massive monster. There was a grip, a hilt, sticking out of the Sorting Hat, the bundle that Fawkes had dropped. As he dashed for it, the phoenix took off in the other direction, landing by the injured Hermione, and weeping tears onto her wound. The burning of the venom intensified for a few moments, before being replaced by a soothing coolness. The wound closed rapidly, not even leaving a scar.
"NO!" roared the diary's projection, as the catboy dragged a gleaming, silver broadsword from the hat, and turned in time to thrust it above him as the basilisk's mouth engulfed him. In a spray of blood, the end of the sword emerged from the roof of the basilisk's skull, and the huge serpent began to thrash in its death throes, almost spitting Harry back across the room. The wounds he received from the deadliest of snakes were more serious than his mate's, however. His right shoulder had been pierced by one of the basilisk's fangs, which was still embedded in the injury, while a second fang and opened up the scar on his brow, burning it with such intensity that Hermione thought she saw a flare of greenish smoke. As Harry screamed from the agony, she could have sworn she'd heard a second scream, smaller, yet of much higher pitch. As Fawkes wept onto her mate's injuries, she wrenched the fang from his shoulder.
'Riddle' was speechless. These half-human abominations (as he saw them, at least) had killed a millennial basilisk. It was impossible. Hermione kicking the diary out of Ginny's hand dragged his notice back to the possibility of... mortality. As she started hurling every destructive spell they'd been taught, as well as few from her 'independent study', the sneer came back to his face.
"There's nothing you can do, freak," he smirked, not noticing Harry's eyes narrow to slits, nor the way his ears laid flat back in rage. "I charmed that to be impervious to all spells that you can think of."
The emerald-eyed feline struggled to his feet, limping forwards. "For the last time Riddle," he growled as he picked up the fang that had been in his shoulder, wrapping it with the edge of his robes, just in case. "Shut. The. Hell. Up. And. Die." Each word was punctuated with a stab of the fang into the diary, which screamed and fountained some kind of ichor, too thick to be ink and too black to be blood. Each strike with the fang brought a corresponding hole in Riddle's projection, and the holes burned with a blackish fire as they consumed the spirit within the diary.
The two felines collapsed into each other's arms, trembling as the adrenaline began to wear off, and licking at each other, grooming their fur. Upon discovering that the basilisk blood tasted extremely nasty, they agreed that a shower would be better.
It was there that the teachers found them, the two exhausted felines and the girl they had gone to save.