Harry snuck out of Gryffindor tower under his invisibility cloak and headed to Moaning Myrtle's bathroom. When he entered he saw no sign of her, and hissed open at the right sink. The entrance to the Chamber of Secrets opened and he slid down the pipe. His feet crunched on the bones of rats and other small animals as he landed on the floor of the Chamber. He continued through the Chamber until he reached the large area with the statue of Slytherin's head. Are you there? I heard you speaking earlier. Harry hissed as loud as he could. He heard scales sliding to his left, and turned to look at the basilisk that had entered the area, remembering to keep his eyes down of course.
Do not worry. I have my second eyelids closed, so you may look at me without fear.
Why are you looking for a mate?
There is no one for me mate with and I am lonely. But you, Speaker, will do nicely.
What? Ah! Exclaimed Harry as the basilisk lunged forward as fast as a whip and bit into his shoulder. Harry screamed as pain filled him up to brim and writhed on the ground, the basilisk's fangs still buried in his shoulder.
Gradually, the pain receded, and Harry was left panting on the ground. His body felt sore and unusually heavy. He turned to look down and almost fainted. He was naked, and now a 16-foot snake from the waist down. He was a lamia! A male, granted, but a mythical half-snake half-human nonetheless.
What did you do to me!?
I made you my mate. Now we will both be content and stay by each other's sides for eternity.
Harry couldn't bring himself to argue. He felt inexplicably drawn to the basilisk, and felt a growing need to feel those scales against him.
What is your name? I can't keep thinking of you as "basilisk."
My name is Hyssop. Now, we mate. Hyssop replied, slithering around behind Harry. He gasped as Hyssop flicked his tongue over Harry's neck and rubbed his scales against him. He could feel himself grow wet with self-made lubricant as the snake rubbed his member against the opening hole in Harry's tail. Harry shrieked in pain/pleasure as Hyssop entered him in one swift thrust. The basilisk didn't wait for Harry to adjust and proceeded to pound into him with fervor. Harry moaned in pleasure as his prostate was repeatedly struck. Their tails twined together as they mated, until Hyssop came violently in Harry's warm channel. They both hissed contentedly and separated, Hyssop's tongue flickering over Harry's neck once more before he left to hunt rats for his new mate. They would need a nest, so he would have to let Harry leave to collect materials. But then, Harry would be all his and never leave.