BTVS and the Harry Potter Universe belong to Joss Whedon (Mutant Enemy Productions) and J.K. Rowling respectively.
AU- Some Canon, but otherwise extremely AU. Tried to keep to the basic timelines of Harry Potter Universe, some things have been changed though to better fit the story.
Rated R for some violence and language
How the Light was Extinguished By: LionessHeart
It was a dark and cold night when Merope Gaunt, or as she called herself in her mind Merope Riddle, stumbled up the steps of the muggle orphanage with two bundles in her arms. When Mrs.Cole, the proprietress of the place, answered the door, she almost shrieked in fright. The woman had the appearance of a corpse, who was holding onto something precious before it would let itself disintegrate. Merope gave the two bundles to Mrs. Cole, who gave a rare smile at how gorgeous the two children were, the boy with his black peach fuzz and blue eyes, and the girl with what would grow to be golden blonde locks, lighter than her mother's, and light blue eyes mixed with brown. The woman said she hoped the boy looked like his papa, and that he should be called Tom for his father, and Marvolo for her father. She hoped the girl would look like her mother, because as Mrs. Cole would say at a different time to a different wizard, Merope was no beauty herself, and that she should be called Elizabeth Salazara; she said both their surnames would be Riddle, and that she was leaving Tom a ring with the family crest on it and Elizabeth the emerald locket she was wearing. She then died soon after without another word, and the artifacts were sold by Mrs. Cole and wouldn't be seen again for a very long time.
-Quotes taken from Chapter Thirteen "The Secret Riddle" from Harry Potter and the Half-Blood Prince