Disclaimer: I do not own anything of the Harry Potter franchise that seems familiar.
Prologue
Christmas Break 1995
Most of Number Twelve Grimmauld Place's residents were having a gloomy Christmas with Mr. Weasley in the hospital. Harry Potter was exploring a part of the house he was not in the last time he was there. This part of the house seemed untouched by Mrs. Weasley's cleaning. He saw a door that had "Cassie's Room" carved into it. Curious, he turned the door knob and found it unlocked. Inside was a room decorated the way he would image a Slytherin girl would decorate it, a lot of green with elegant furniture.
"You know you aren't supposed to be in this part of the house," Sirius said from behind him. Sighing, he stepped into the room. "What's sad is we never found out whose side she was on. Voldemort did kill her personally but he was killed his own followers in the past."
"Who?" Harry asked.
"Cassiopeia Lyra Black, my younger twin sister. We were quite close before Hogwarts. She would have made an excellent Marauder. We grew more and more apart during our years at Hogwarts." Sirius walked out of the room into a room with his own name on it and closed the door behind him.
Harry took one last look at the room and walked out closing the door behind him. He went back to his and Ron's room where the pair of them played chess watched by Hermione, Ginny, and Crookshanks.
AN: This is a story I started years ago that I randomly decided to continue working on since I am having slight writer's block on my other story. Although, it has been less than half a year since I updated my other story which is not bad since I have gone almost a year without updating it before.
The rating might be a little extreme but the themes in this story do get quite dark.