Plot: Harry is raised by Sirius and Bellatrix, and learns to hate the light side that pursues them. Highly AU, slash in future chapters won't say who, though!
Characters: Harry Potter, Sirius Black, and Bellatrix Black/Lestrange almost exclusively for the first few chapters.
Author's Notes: This story is going to be AU. Really, really AU. But the change from canon comes from some very small differences. In canon, Bellatrix Lestrange, Rudolphus Lestrange and Rabastan Lestrange go torture the Longbottoms just after Voldemort's demise, trying to get information on where he is. Personally, I've always thought that the torture, and subsequent imprisonment, were what caused her to go insane. In this story, Voldemort has given one of his most trusted servants the job of eliminating the Longbottoms, and their son, Neville - because even if he found Harry more of a threat, it never made sense to me that he hadn't considered Neville at all.
Second, Bellatrix Black, according to canon, gets married shortly after graduating from Hogwarts, to Rudolphus Lestrange. Althought I am following this, I am making it a very definite marriage of duty. Bellatrix and Rudolphus will not live with each other, and they will be married in little less than name, barely knowing each other.
This is the revised version of the chapter, with better characterisation, I hope. Thanks for the patience!

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The old-fashioned lampposts along the cobblestone street glowed cheerily. A tall figure blinked into view, suddenly, and began to walk down the street. As it passed under the lights, a sharp-featured woman's face was illuminated, with full pink lips. There was a feral edge to the set of her brows, and a desperate sort of glint in her dark eyes.

She stopped in front of one of the silent houses and read the number on the fence carefully. Setting her jaw and straightening her back, she stepped through the gate, then up a path of worn stones which turned into steps. She drew up to the front door and stopped, tossing back the hood of the long cloak she wore.

Inside, muffled through the door, she could hear a baby's excited laugh. "Good job! Come on now, walk to daddy! Come on, Nevvy!" someone cooed.

Her hand hovered over the doorknob for a long moment.

She opened it.

The next few minutes were a blur of screams and crying and spells tripping from her lips. It was all over too soon for her; if nothing else, she loved a good duel. But they were quickly subdued. The man lay silent on the floor, Stupefied. His wife was passed out - her legs were bent at an odd angle; Bellatrix thought she might have broken them but she wasn't sure. And the baby - the baby was screaming, plump face screwed up and fingers halfway in his mouth, nose running all over the place.

"Kill the child first" - those had been her instructions. She stepped closer to him where he sat on the carpet, and he began to hiccup from the strength of his tears.

She stared at him for a minute... he was so young. She raised her wand, and he started to cry even more loudly - she stiffened. Then, angrily, she turned and ran out the door, leaving it open as she apparated in the split second between her feet leaving the steps and touching the ground.

Her body reappeared in an even quieter neighborhood, and she strode determinedly until coming to a house with a cheap "For Sale" sign in the yard. Sitting down on the front steps, her head in her hands, she tangled her fingers in her dark hair. The street lights flickered out, and she jumped and then cursed herself for a coward. She tapped her wand with one of her fingers, deftly, as if it were a cigarette, and the end lit immediately, giving her pale face and dark hair a ghostly, blue-tinged glow.

She couldn't do it. The consequences, of course, would be dire - she winced, thinking of the Cruciatus she had endured on her initiation night, the memory from long ago but as clear and sharp as if it was happening again as she sat there - but she could not kill them. He will be angry, very angry, she thought, and fear flooded her. She sat for a moment, looking out at the darkness.

Then she gasped and clutched her forearm, her wand dropping forgotten to the ground. She had to bite her lip to hold back a scream -

She pulled back her sleeve, blocking out the pain with a shudder. The mark - the sign of her service to Him - was burning, making the design stand out even more from her pale arm. A skull, with a snake curled about it; it had taken all her reserves not to scream as he burned it there.

Then the pain stopped, and she gasped with the sudden relief. Still throbbing dully, the mark began to fade. That's not right, she thought frantically, and she clenched her arm, staring at the vanishing mark. Was He doing it? Had He known her disobedience?

"No!" she hissed, panicked, scooping up her wand as she stood - she would beg for his forgiveness, her Lord would forgive her, he had to!

She turned on her heels and disappeared.