Disclaimer: I do not own either Harry Potter or Percy Jackson and the Olympians. I believe JK Rowling and Rick Riordan own them, but I am not quite sure.
October 31, 1981
It was a Halloween night. Most inhabitants of Godric's Hollow already returned to their homes, because even simple mortals – simple Muggles – felt, perhaps consciously, that something stirred in the night. They felt this every year, and for good reasons – there were things in the dark, some old, some new, but all were not to be seen.
A pale creature in long black robes moved in a village. Two children dressed as pumpkins were waddling across the square. And the once-man was gliding along, content with sense of purpose and power and rightness in him that he always had on these occasions… He believed himself above anger. That was for weaker souls than he, but triumph, yes… He had waited for this; he had hoped for it, he did, after all, expect triumph.
"Nice costume, mister!"
He saw the small boy's smile falter as he ran near enough to see beneath the hood of the cloak, saw the fear cloud his painted face.
Then the child turned and ran away. Beneath the robe be fingered the hand of his wand. One simple movement and the child would never reach his mother… but the creature thought it quite unnecessary, and so the boy ran, as did the countless thousands of thousands when faced with something different from them.
And along a new and darker street he moved, and now his destination was in sight at last, the Fidelius Charm broken, though they did not know it yet. And he made less noise than the dead leaves slithering along the pavement as he drew level with the dark hedge, and peered over it.
They had not drawn the curtains; he saw them quite clearly in their little sitting room, the tall black-haired man in his glasses, making puffs of coloured smoke erupt from his wand for the amusement of the small black-haired girl in her blue pyjamas. The child was laughing and trying to catch the smoke, to grab it in her small fist.
A door opened and the mother entered, saying words he could not hear, her long dark-red hair falling over her face. Now the father scooped up the daughter and handed her to the mother. He threw his wand down upon the sofa and stretched, yawning.
The gate creaked a little as he pushed it open, but the man did no hear. The creature's white hand pulled out the wand beneath his cloak and pointed it at the door, which burst open.
He was over the threshold as the man came sprinting into the hall.
It was easy, too easy; he had not even picked up his wand…
"Lily, take Rose and go! It's him! Go! Run! I'll hold him off!"
The man was a fool, for he, unlike Muggles, relished in the power given to him by a simple focus of power, and it filled him with overconfidence. The creature laughed before casting the curse.
"Avada Kedavra!"
The green light filled the cramped hallway, it lit the pram pushed against the wall, it made the banisters glare like lightning rods, and the man fell like a marionette whose strings were cut.
He could not hear her screaming upstairs. That surprised him. No, he heard a chanting, an ancient chanting which was as old if not more than the one he used to make his Hocruxes. She chanted in Latin and Greek, weaving the words together in a prayer…
His eyes widened.
He forced the door open, cast aside the chair and boxes hastily piled against it with one swift wave of his wand . . . and there she stood, the child in her arms. At the last sight of him, she dropped her daughter into the crib behind her and pulled out a knife of black metal. She finished the chanting and now he could make out a circle on the floor around the crib. She shouted a single word.
"Hades!"
"Stand aside, you silly girl . . . stand aside now."
But she ignored him, and made a single cut on her hand, letting blood flow in the circle.
He screamed in fear and frustration.
Green light flashed around the room and she dropped like her husband. The child had not cried all this time. She could stand, clutching the bars of her crib and she looked up into the intruder's face with a kind of bright interest, perhaps thinking that it was her father who hid beneath the cloak, making more pretty light, and her mother would pop up any moment, laughing—
He pointed the wand very carefully into the girl's face. He wanted to see it happen, the destruction of this one, inexplicable danger. The child began to cry. It had seen that he was not his father. He did not like it crying, he had never been able to stomach the small ones whining in the orphanage—
"Avada Kedavra!"
And then he broke; He was nothing, nothing but pain and terror, and he must hide himself, not here in the rubble of the ruined house, where the child was trapped and screaming, but as far away as possible.
A few hours later
A young man by the name if Severus Snape stood on the ruins of the house. He came here to see if Lily was alive. He hoped with all his heart though he knew, with his mind, that the Dark Lord would not spare her. He still hoped, but…
The weather matched his mood. The rain fell on the charred stone, on the burnt grass, on the broken trees and on the kneeling man. He wept, letting go of his Mind Shields for the first time in some years. He whined like a dog whose master left it and never returned. He whimpered like a broken animal, not quite dead but definitely not alive. He stayed like that for more than an hour, letting his grief pour out. Lightning flashed in the sky and, immediately, thunder followed.
Severus heard a child crying in the rubble. He never would have thought that it would survive the destruction of the house, but it did. And it was the last piece of Lily left in this world. He staggered towards the sound. Behind a piece of the wall he saw a girl, lying in a crib, surrounded by a circle of blood. He gasped. He knew, of course, that Lily read dark texts even back in Hogwarts, after the Fifth-Year-Incident, but still, he would never expect Lily Evans to use blood rituals to protect her daughter. This was really dark magic, and extremely obscure. He only heard of it once from the Dark Lord, when he ordered the younger Black to bring a tome about it from the Black Manor. The man had no idea where Lily found out about it, but it most certainly saved her girl.
Severus took another step to have a closer look at the girl. She had black hair, pale skin, and eyes that looked exactly as Lily's. Absolutely the same. It was really unnerving. The guilt of betraying Lily hung so heavily on him and twisted his mind so much that he could think of only a single way to repay his debt – to raise her daughter. He knew he would berate himself for this decision later, and he also knew that the Dark Lord was not truly gone – for the Dark Mark was still on his wrist, even though it was not black but dark grey, somewhat… faded. Severus hoped it would stay this way. He picked up the child from her crib, and, while the brief episode of insanity still lasted, Disapparated away.
When a huge man came to Godric's Hollow he found only two bodies and the ruins of a building.
He also cried.