Disclaimer: Sadly, I don't own Harry Potter.
It was night at Hogwarts, School for Witchcraft and Wizardy. Students slept or hurriedly wrote their homework for tommorow. Teachers slept, corrected homework in a hurry or patrolled the corridors.
Only Tom Marvolo Riddle was awake in both body and mind. He carefully examined the portrait before him. "Specialis Revelio", he wisphered.
Nothing. His face showed no dissappointment as he used more advanced and more complicated methods of magical detection. His noble ancestor would not have made the chamber easy to find.
Finally, he moved on. Within five years, ever since he had heard the myth, he had investigated even the faintest hint. He had spoken to all the snakes in every portrait. He had checked every Book in the library, including the restricted section. Then he came to the conclusion, that it would be more prudent to just search Hogwarts systematically, from bottom to top.
He looked at his watch. In ten minutes he would have to hide from Professor Slughorn on his patrol. Then he would continue his search for another thirty minutes, before retreating back to bed before, Dumbledores watch began.
Tom Riddle, or Lord Voldemort as he liked to call himself, stopped in front of a small door. Surely Salazar Slytherin would not have placed the entrance to the Chamber of Secrets in a girls toilet ? Then again who would search a girls toilet for Salazar Slytherins secret chamber ?. He was famous for his cunning, after all. Tom sighed and opened the door. At the very least he could hide from Slughorn in there.
He spotted the tiny snake immediately. After years of searching he had gotten quite good at it. He had dreamed of tiny snakes, hidden in a plain, innocent spot. If he closed his eyes he could see snakes dancing in front off him. Sometimes he could see them even if he didn't close his eyes.
"Hello, little brother" he hissed. The snake did not respond. This was truly most peculiar. All the other snakes had responded, even if the response wasn't worth anything. Besides, what was a tiny snake doing in a girls toilet. This had to be the chamber. On the other hand ... woudn't Slytherin have used something that looked more innocent ? A normal snake, until the keyword was spoken ?He was known for his cunning after all. But maybe that's exactly what he wanted them to think, that he would never be so blunt to make his biggest secret so damn obvious. Tom sighed heavely and went back to work.
"Specialis Revelio, Specialis Revelio, Invenin, Invenin, Exhibio, Exhibio ...". He went on and on and on ,but still all his results pointed towards a normal bathroom.
Turning towards the snake again he said "Speak to me !". Nothing. "Salazar Slytherin's heir commands you: Open ...". He had barely finished the word when the sink in front off him was replaced with a dark hole. With only a seconds hesitation, he jumped in.
What exactly would "The horror within" be ? Maybe full control off the Hogwarts wards at a level higher then the Headmaster, with a few secret, more nasty wards on top of them. Hopefully, because it would be so fun to throw Dumbledore out. Or maybe a nest off venomous snakes, keyed into the wards and with a method off travelling undetected. He would set one on every single mudblood in Hogwarts. Then he would breed more and set them on the survivors and his enemies. And then he would set them all on Dumbledore.
Or maybe it was his ancestors journal, his findings and procedures. All his knowledge on Potions, Legilimence and even the Dark Arts. Maybe even – dare he hope – the secret of immortality ? Surely Slytherin himself would not be content with a short, mortal, life.
Tom Riddle came to a hold. It looked like the end, but he knew better. "Open in Salazar Slytherins name". It felt good when when the wall shifted. Like he had finally been aknowledged for what he was: The most brilliant student Hogwarts had ever had. The heir of Slytherin. The one who would put muggles and mudbloods in their place. In one word: Lord Voldemort.
He stopped, Slytherins tall statue towering over him. "Your heir requires an audience", he said.
The answering silence was painfully obvious. "Open" he tried again. "I will rid this world off all mudbloods and blood-traitors".
This time the silence was not only painful and obvious, but most of all embarassing.
"Speak to me", he shouted angrily, "Speak to me, Slytherin, greatest of the Hogwarts four".
With relief he saw the statue open it's mouth. Of course, there was no way Lord Voldemort would fail after he had come so far. He saw a huge, forked tongue shoot out of the hole.
Then the head of a gigantic snakewith yellow eyes came out of the the statues mouth.
Two thoughts flew through Tom Riddles head. The first was "It's a Basilisk!" the second "I looked into it's eyes ...". And then, he died.
"What a pity", the gigantic snake hissed, "I was looking forward to some company. It got a bit lonely after the first couple of centuries".