Harry Potter and all related characters and ideas are the property of J.K Rowling. Bloomsbury, Scholastic, Warner Brothers, and other people with more money than me.
Yes, it is an OC. Whether or not she becomes a Mary-Sue remains to be seen, but I will try to avoid it.
Prologue
Steel colored clouds hung low over the stone fortress, a light but unrelenting rain fell and shrouded the entire island of Azkaban in a pale mist, hiding it from muggle eyes, but also not allowing its inmates even the slightest glimpse of the choppy grey waves that might stir their memories of the world that existed beyond.
Inside, in a dank, dark cell that never saw a glimpse of daylight or a breath of fresh air, a man leaned against the stones, and listened to the screams of the other prisoners that were loud enough to penetrate the walls. He did not know how long he had been there, it might have been only a few weeks, or twenty years. In Azkaban there were no days and nights, only the darkness and chill all the time. Only that, and the constant presence of the Dementors in his mind, reminding him of the mistakes he had made, providing him with images of a scene he had not actually witnessed, the deaths of his best friends, and over and over again seeing a scene he had witnessed- "James and Lily Sirius! How could you?" and then the explosion of fire and flying concrete, screams, and blood…
Another scream, a woman's, penetrated the walls, followed by a sound that did not belong in the walls of Azkaban Prison…the high, thin wail of an infant. Sirius Black wondered a moment, for this was a place no child should ever be, but seconds later the sound was gone, and he wondered if it was the madness setting in. He thought, perhaps, the madness would be a blessing. He knew the dementors were coming, he couldn't hear them, but he could feel them. Summoning what little strength he had, the man let go of such human thoughts, and transformed, settling stretched out on the stone floor, and hoping for the only refuge that prisoners in Azkaban might seek- sleep.
In another cell not so far away, a woman lay, exhausted and shivering. For Bellatrix, the madness had come quickly, but the physical pain of that day had left her more lucid than she had been since she had come to Azkaban prison. Bella wondered why she felt the dementors more keenly than most. Perhaps, she was darker than most, had done more to remember. She had not felt guilt then, and nor did she now, but nonetheless they used those things to torture her. Bella knew only one thing, that she kept always in her mind. Her master would return, that was what had kept her from succumbing to the lure of surrender, of death, like so many others.
Now, there was another reason she would go on, even as the madness clawed at the edges of her mind, she stared at the door of the cell, where the dark-cloaked witch from the ministry had taken the child. Her child. When the Dark Lord returned, when he released her from this Hell, she would find the child. A girl. A smile twisted her features, no longer as beautiful as she had once been, turning gaunt already, as she thought about the little girl. A witch, of her blood, hers was a family of proud, powerful women. Yes, the child would be great. Bellatrix only had to wait, until the Dark Lord came for her. He would reward her, his most loyal servant, beyond her wildest dreams, and she would be reunited with Rudolphus, reunited with their daughter. With that day in mind, one thing the dementors could not take, she closed her eyes, hardly feeling the pain of her body, but instead seeing in her mind the baby's grey eyes she had in reality seen for only that split second. Her daughter.