This isn't a one-shot, it's an idea for a story I came up with after i read the Barimaeus Trilogy.
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London: December 21, 1858

Kairi opened her eyes. Flashes of light were visible under the door. She sat up and rubbed the sleep from her eyes. She swore she could've heard chanting coming from the sitting room.

She kicked the covers off and crept out of bed, stepping over to the door. Kairi pressed her body flat against the wall, reaching out to the door with a pale hand she pushed it open. She peered out, her red hair falling in curtains into her face. She huffed, pushing a lock behind her ear and started to tip-toe down the hall.

As she neared the sitting room, the chanting grew louder and a flash of red light briefly illuminated the hallway. Kairi froze in terror, like a deer cornered by hunters. A moment passed, she hadn't been seen. She let out a breath of relief and and poked her head into the room. She gripped the door frame, her knuckles white with fear.

Kairi saw a man facing away from her, his arms extended, his hands holding balls of the eerie light. Kairi's eyes were drawn to a writhing figure on the floor. She stifled a gasp. Her mother was sobbing with pain as light surged through her. Kairi saw the man pull a knife from somewhere within his coat. Her mother looked up at her tormentor.

"Willian, please," her mother begged. "Don't do this,"

William? Something clicked in Kairi's mind. The man towering over her mother was her father. She backed up into the hallway. I have to get help, she thought.

Her mother screamed as William plunged the knife into her chest. Kairi's eyes brimmed with tears. As she watched her mother's dying form she noticed something else, a crude pentacle had been drawn on the floor beneath her mother.
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That's all I have for now, I'm starting to have a small inkling of an idea that I may use to continue this, tell me what you think!